The Scala compiler has to be cleaned up. Maybe rewritten, maybe not. Dotty started as a fork of Scala, and now Odersky decided/announced that it'll be merged back eventually. (And it should conform to whatever the Scala language is/will be after the SIP processes for 3.0.)

I've never seen a system or project that's had a "big" change that wasn't always a total rewrite.

I guess it's always envisioned differently like "can we tackle this subsystem in isolation".

The old saying: "Its bigger and slower, but at least its more expensive!"

My experience says, a rewrite just has different bugs. Combined with delay and cost, its often not worthwhile.