WPF ListBox DataTemplate MenuItem MVVM

//xaml
<Window x:Class="WpfApp212.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp212"
        mc:Ignorable="d" WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <Style TargetType="{x:Type TextBlock}" x:Key="tbStyle">
            <Setter Property="FontSize" Value="20"/>
            <Style.Triggers>
                <Trigger Property="IsMouseOver" Value="True">
                    <Setter Property="FontSize" Value="40"/>
                    <Setter Property="Foreground" Value="Blue"/>
                </Trigger>
            </Style.Triggers>
        </Style>
        <Style TargetType="ListBoxItem">
            <Setter Property="FontSize" Value="30"/>
            <Setter Property="Foreground" Value="Black"/>
            <Style.Triggers>
                <Trigger Property="IsMouseOver" Value="True">
                    <Setter Property="FontSize" Value="80"/>
                    <Setter Property="Foreground" Value="Blue"/>
                    <Setter Property="FontWeight" Value="ExtraBold"/>
                </Trigger>
                <Trigger Property="IsSelected" Value="True">
                    <Setter Property="FontSize" Value="100"/>
                    <Setter Property="Foreground" Value="Red"/>
                </Trigger>
            </Style.Triggers>
        </Style>
        <Style TargetType="{x:Type MenuItem}">
            <Setter Property="FontSize" Value="50"/>
        </Style>
    </Window.Resources>
    <Grid>
        <ListBox ItemsSource="{Binding PersonsList}"
               Grid.IsSharedSizeScope="True">
            <ListBox.ContextMenu>
                <ContextMenu>
                    <MenuItem Header="Save Selected As Json" Command="{Binding SaveAsJsonCmd}"
                  CommandParameter="{Binding RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=ContextMenu},Path=PlacementTarget}"/>
                    <MenuItem Header="Save All As Json" Command="{Binding SaveAllAsJsonCmd}"
CommandParameter="{Binding RelativeSource={RelativeSource Mode=FindAncestor,AncestorType=ContextMenu},Path=PlacementTarget}"/>
                </ContextMenu>
            </ListBox.ContextMenu>
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto"
                                              SharedSizeGroup="abc"/>
                            <ColumnDefinition/>
                        </Grid.ColumnDefinitions>
                        
                        <TextBlock Text="{Binding Name}"   
                                   Margin="4"/>
                        <TextBlock Grid.Column="1"   
                                   Text="{Binding Age,StringFormat=is {0} years old}"
                                   VerticalAlignment="Bottom" Margin="4"/> 
                    </Grid>
                  
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </Grid>
</Window>

//cs
using Microsoft.Win32;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp212
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            var vm = new BookVM();
            this.DataContext = vm;
        }      
    }

    public class BookVM
    {
        public BookVM()
        {
            InitData();
        }

        private void InitData()
        {
            PersonsList = new ObservableCollection<Person>();
            for (int i = 0; i < 10000000; i++)
            {
                PersonsList.Add(new Person()
                {
                    Age = i + 1,
                    Name = $"Name_{i + 1}"
                });
            }
        }

        public ObservableCollection<Person> PersonsList { get; set; }

        private DelCmd saveAsJsonCmd;
        public DelCmd SaveAsJsonCmd
        {
            get
            {
                if (saveAsJsonCmd == null)
                {
                    saveAsJsonCmd = new DelCmd(SaveAsJsonCmdExecuted, SaveAsJsonCmdCanExecute);
                }
                return saveAsJsonCmd;
            }
        }

        private bool SaveAsJsonCmdCanExecute(object obj)
        {
            return true;
        }

        private void SaveAsJsonCmdExecuted(object obj)
        {
            var lbx = obj as ListBox;
            if (lbx != null)
            {
                string selectedItemJson = JsonConvert.SerializeObject(lbx.SelectedItem, Formatting.Indented);
                SaveFileDialog dialog = new SaveFileDialog();
                dialog.Filter = "Json Files|*.json|All Files|*.*";
                if (dialog.ShowDialog() == true)
                {
                    File.AppendAllText(dialog.FileName, selectedItemJson);
                }
            }
        }

        private DelCmd saveAllAsJsonCmd;
        public DelCmd SaveAllAsJsonCmd
        {
            get
            {
                if (saveAllAsJsonCmd == null)
                {
                    saveAllAsJsonCmd = new DelCmd(SaveAllAsJsonCmdExecuted, SaveAllAsJsonCmdCanExecute);
                }
                return saveAllAsJsonCmd;
            }
        }

        private bool SaveAllAsJsonCmdCanExecute(object obj)
        {
            return obj != null && (obj is ListBox);
        }

        private void SaveAllAsJsonCmdExecuted(object obj)
        {
            var lbx = obj as ListBox;
            if (lbx != null)
            {
                var allItemsJson = JsonConvert.SerializeObject(lbx.Items, Formatting.Indented);
                SaveFileDialog dilaog = new SaveFileDialog();
                dilaog.Filter = "Json Files|*.json|All Files|*.json";
                if (dilaog.ShowDialog() == true)
                {
                    File.AppendAllText(dilaog.FileName, allItemsJson);
                }
            }
        }
    }

    public class DelCmd : ICommand
    {
        public event EventHandler CanExecuteChanged
        {
            add
            {
                CommandManager.RequerySuggested += value;
            }
            remove
            {
                CommandManager.RequerySuggested -= value;
            }
        }

        private readonly Action<object> execute;
        private readonly Predicate<object> canExecute;

        public DelCmd(Action<object> executeValue,Predicate<object> canExecuteValue)
        {
            execute = executeValue;
            canExecute = canExecuteValue;
        }

        public DelCmd(Action<object> executeValue):this(executeValue,null)
        {            
        }

        public bool CanExecute(object parameter)
        {
            if(canExecute==null)
            {
                return true;
            }
            return canExecute(parameter);
        }

        public void Execute(object parameter)
        {
            execute(parameter);
        }
    }

    public class Person
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }
}