That's assuming you don't separate concerns in your monolith into modules with clear boundaries and dependencies. If you did that from day one, splitting a module off to a different service is easy.
And Service Oriented Architecture is the nice middle ground of that infrastructure axis
The history of distributed software is littered with the corpses of attempts to make the RPC look like a function/method call (I worked for a CORBA vendor decades ago). But these two techniques are so different on a fundamental level they are not substitutable except in the most trivial cases. The compute and elapsed time RPC overhead which makes perfectly good local abstractions completely impractical across a network. And then there are the differences in terms of failure modes, concurrency and synchronicity.
So even a beautifully designed and layered piece of “monolithic” software with a code base costing of coherent and clean abstractions will not be easy to port to a microservices/distributed architecture.