> you end up with both the old monolith and new microservices, the monolith never gets fully broken up, and now you need to support two development paradigms forever.

It's not a bad thing, it's actually a quite pragmatic approach. Some folks call it The Citadel:

https://m.signalvnoise.com/the-majestic-monolith-can-become-...

Building monolith is the best way to learn about domain and domain boundaries. Monolith is way more forgiving for your mistakes.

Going all in on microservices is like believing you learned everything there is. Some folks believe that, most of the time they are wrong.