Several people have expressed similar, implying that it’s better to be compatible with WP, as if that is the only reason to backport from WP.
Let me just be clear that the beginning of CP had several developers with WP core experience. James was the last of those, and with him gone, the CP team won’t win any resume contests. The whole point of backporting from WP is because they have thousands of developers, millions of users testing every combination of version and plugin and host to find problems (plus a testing team), a security team, and a performance team. CP has none of that and it’s kind of silly to not take advantage of their efforts. But the more things we ignore or fall behind on, the harder it is to backport anything.
There are many things that continue to evolve, outside of WP, like PHP, Javascript, CSS, HTML, and various bundled tools (like jQuery and TinyMCE and PHPMailer and Simple Pie and Requests…).
CP can’t stand still at 4.9. That’s dead. But if you tried to backport all the PHP8 stuff, you’d find it very difficult because of all the formatting changes they made, plus all the bug fixes, plus all the new features.
The new fork bypasses the backport problem by taking it all at once and deleting the block stuff that is unwanted.
I personally think that CP doesn’t have any features of value that WP doesn’t have. It has a bunch of fixes and a few features from WP, but it’s a dead end, especially with the limited roster of people who contribute code.