With Rosetta users will be at Apple's mercy. Sure, initially there will be lots of reasons to make it very good but in a couple of years from now there will be lots of pressure to support an ARM-only world.
Besides, whilst most software developers will probably make the shift to ARM fairly quickly – here Apple has done a lot of work in making cross-compiling easy and fast – it's often the less interesting bits but equally important bits of software that won't be updated. For example, switching off 32-bit x86 put paid to quite a few things like my printer controller and I'm routinely being warned that thinks like my Cisco AnyConnect (from last year) soon won't work either. The way Apple has handled these changes smacks of arrogance and doesn't bode well for future changes.