Why??

What was wrong with the old editor? – Its a pain for users who want more than the bare minimum (Just writing text and maybe click some limited blocks together)

The old variant was fine for both, 80 year old granny and power users. You can add plugins for everything else like content / page builders.

An other thing I do not understand why it is an improvement make the drill down into options more complicated and to reduce the editor area to 40% of my screen estate?

Why? … There are so many other things like the monolithic comment system, performance, security etc. where creative improvements and development efforts are always useful and appreciated I think.