The decision to ditch their engine is very interesting as their tech is pretty amazing has has traditionally been a hallmark of their games.
There is a big issue with hiring across the whole software industry including games, so using Unreal 5 might make it easier to hire new people. This might be exacerbated for them by being in Poland rather than say California.
Also using Unreal could help cut down on development time, as presumably they might want to get a new game out in less than six years.
I am of course very excited for a new Witcher. People are saying the logo looks like a lynx so it might star a witcher from a different school.