That's... not how it works.... If that were true, then moving to a microservice architecture they'd need 5 million instances of the microservice that replaces

That's... not how it works.... If that were true, then moving to a microservice architecture they'd need 5 million instances of the microservice that replaces the part of the monolith that required 5 million instances in the first place, plus a couple of millies for all the other services.