>Perhaps the frontier models of 2026H2 may be good enough to start compacting and cleaning up entire codebases

I don't think this will happen - or rather I don't think you can ask someone, human or machine, to come in and "compact and clean" your codebase. What is "clean" code depends on your assumptions, constraints, and a guess about what the future will require.

Modularity where none is required becomes boilerplate. Over-rigidity becomes spaghetti codes and "hacks". Deciding what should be modular and what should be constant requires some imagination about what the future might bring and that requires planning.