Yes, you can always rewrite parts of a monolith, what I am saying is that it is just way easier to do with the hard-boundaries and well-defined interface contracts of microservices. You know that as long as you live up to the interface contracts, any changes you make to the microservice app cannot adversely affect other microservices. Just think about how much of software development patterns and best practices are devoted to creating proper encapsulation and avoiding tight coupling; All of that is simply built-in when you use microservics.
Yes, of course if you have great test coverage making changes to a monolith can be easier, but good test coverage and debt-riddled don't often apply to the same codebase! Lets be honest, if you are happy with your monolith codebase then there is no reason to rewrite for microservices. It is for those projects that you are not happy with the monolith code, where maintenance of the code IS scary, that the conversion makes sense.