CKEditor 5 is licensed under the GPL 2+ while XWiki is using LGPL 2+. Including CKEditor 5 would therefore change the license of the XWiki distribution to GPL 2+ according to my understanding (I’m not a lawyer). Therefore, it is very unlikely that XWiki will ever ship with CKEditor 5 as it’s core editor. Switching to another editor seems way more likely as the way forward.