[samus]

... was taking years to land Project Loom. So long that people started calling it vaporware. Project Valhalla is still regarded as such by many. It had to be shipped as soon as it was usable, even though a few rough edges remain that really ought to be deburred.

As you indicate, the complexity lies in not burning too many bridges with existing users and use cases. This is something that Android regularly does and which Go never really had to do due to its shorter history and up-front design.