What same-process modularization does _not_ solve is independent lifecycle management. In a monolithic system your change rate often becomes tied to your slowest, most bug prone module or team. If you've some integration test that bakes some piece of important code for 48 hours, you can only make a change _everywhere_ else every 48 hours.
Now sometimes folks (the Windows team famously did this) build systems which identify which tests to run based on changes that occur in the codebase, but that's _not_ easy.
Now sometimes folks (the Windows team famously did this) build systems which identify which tests to run based on changes that occur in the codebase, but that's _not_ easy.