Really, the development methodology you pick depends on your set of requirements.
Monolithic applications are more difficult to change, and generally are more fragile.
Microservice-based applications have their own challenges, but because the APIs are published their contracts are much easier to enforce and side-effects are essentially non-existent...or at least easily traceable.
You can sort of get modularity in a monolithic app, but you start having versioning problems once you get to a certain point. You can design around that problem in a microservice architecture, since it's the behavior that matters, not the implementation.
In the end, micro services will probably be better because it reduces the cognitive load on an individual developer. It requires more work up front, but in the long run developer brainpower is the most expensive resource you have, so anything that makes development easier will win.