Migrated a Scala 2.8(!) era codebase to Scala 3.

As OP explains, macros and abstract type projections tend to be the biggest pain points in complex applications; otherwise, with Scala Rewrite tool it's pretty straightforward.

I think it's more inertia than anything else that more Scala 2 companies don't migrate.

Unpopular opinion, but setting a Scala 2 sunset time would spur companies into action :)

As it stands Akka (previously Lightbend, previously TypeSafe) is the maintainer of Scala 2, and derives part of its revenue from Scala 2 support contracts so there's even less incentive to migrate when there's no EOL date as Python 2 (eventually) had.