"If we clean up now, won't it just get dirty again?" -> Yes

... but that's not a reason to skip cleaning.

Every factory shop floor gets dirty throughout a shift. The mess is a byproduct of work getting done.

Your Org works the same way: New product lines and reorgs create new Fields, Permission Sets, and Flows, and some of it will outlive its usefulness within a year. That's not a governance failure. That's a business operating.

So the question for leadership is who is keeping the Org clean, and how often.