I think you vastly overestimate the separability of these systems.
Picture 10,000 lines of code in one method, with a history of multiple decades.
Now picture that that method has buried in it, complex interactions with another method of similar size, which is triggered via an obscure side-effect.
Picture whole teams of developers adding to this on a daily basis in realtime.
There is no “meaningful progress” to be made here. It may offend your aesthetic sense, but it’s just the reality of doing business.