Working at a huge company I feel like every service owning its own data store (or at least only accessing/manipulating another’s through its API, not directly in the underlying store) is the only sensible way to do it, because you can never really envision what clients you will have ahead of time or what they will want to do, and that’s an easy recipe for headaches. But the smaller the company gets the further I’d go along the spectrum from full isolation to isolated services with shared stores to monolith.