How to remove list view from media library?

This is a little hacky, but doesn’t require you to edit core files, which you should never do, as you are aware.

add_action('admin_init', function() {
    $_GET['mode'] = 'grid';
}, 100);

This will always force the mode into grid view.

What it does not do,

• it does not remove the List View icon
• it does not change the query argument in the URL

To remove the List View icon you can do something to the effect of:

add_action('admin_head', function() {

    ?>

    <style type="text/css">
        .view-switch .view-list {
            display: none;
        }
    <style>

    <?php

});

Alternatively, to remove the ?mode=list query argument from the URL, you could:

• use JavaScript to modify the search property found on windlow.location
• redirect the user server side using wp_redirect() by inspecting $_GET super global for the mode query variable array key (recommended).

For example you could rewrite the first snippet to:

add_action('admin_init', function() {

    if ( isset( $_GET['mode'] ) && $_GET['mode'] !== 'grid' ) {
        wp_redirect(admin_url('upload.php?mode=grid'));
        exit;
    } else {
        //required by upload.php, handle the case if user just navigates to... 
        //http://www.example.com/wp-admin/upload.php (with no mode query argument)
        $_GET['mode'] = 'grid';
    }

}, 100);

Or alternativey without the need for an else block,

add_action('admin_init', function() {

    if ( strpos(admin_url('upload.php'), $_SERVER['REQUEST_URI']) !== false 
         || (isset($_GET['mode']) && $_GET['mode'] === 'list') ) {
        wp_redirect(admin_url('upload.php?mode=grid'));
        exit;
    } 

}, 100);

…the above snippet ensures that no matter what, either of the following URLs,

• http://www.example.com/wp-admin/upload.php
• http://www.example.com/wp-admin/upload.php?mode=list

…will always be redirected to,

• http://www.example.com/wp-admin/upload.php?mode=grid

Combined with hiding the List View icon via injecting CSS on the admin_head hook, produces the desired result you are after.