Whats the big advantage of Trailing commas are allowed in parameter lists? I never liked it, if its not there, dont do a kind of pseudo initialization.

Maybe not for single line calls, but for multi-line helps with some readability and git diffs.

$result = thing(

    $arg1,

    $arg2,

    $arg3,

);

It makes diffs easier to read, and ensures you never forget to add one of you'd add a new line to the parameter list.

One of important weakness of php language is loosing generic type, I think this is one of most important feature we need in php.

Why a new major version?

It's the standard release cycle of PHP: https://www.php.net/supported-versions.php