> pretty much full rewrite

This is far from my understanding. Changing out this signature scheme is hard work, but doesn't require a rewrite of the VM.

Ethereum is way more complex than let's say Bitcoin and all parts are affected. This is not just the "signature scheme".

The fact that the signature size is multiplied by ~10 will greatly affect things like blockspace (what I guess is even more a problem with Bitcoin !)

Also they are the only blockchain I believe that put an emphasis on allowing large number of validators to run on very modest hardware (in the ballpark of a RPI, N100 or phone).

My understanding is they will need to pack it with a larger upgrade to solve all those problems, the so called zkVM/leanVM roadmap.

And then there are the L2 that are an integral part of the ecosystem.

So this is the greatest upgrade ever made on Ethereum, pretty much full rewrite, larger than the transition to proof of stake. I remember before the Proof of Stake migration they were planning to redo the EVM too (with something WASM based at the time) but they had to abandon their plan. Now it seems there is no choice but to do it.