How can Parmigiano Reggiano keep developing flavor after its starter bacteria have done their early acid-making job?

Answer: Enzymes are released

Salt keeps drawing water — Plausible, but incomplete: salt and moisture affect texture, yet they do not explain why flavor can keep developing after early acid-making slows. The starter activity is part of the make process, and ripening depends on tools already inside the curd. The deeper time story is chemical, not just drying.

Enzymes are released ✓ — Right: early bacteria can shape later flavor after they stop growing. In cheese ripening, starter-culture enzymes contribute to proteolysis, and reviews discuss autolysis and enzyme release as part of that process. The surprise is that the cells do not have to stay lively forever; they can leave chemical tools behind.

Surface flavor seeps in — Plausible, but not the main mechanism: surface conditions matter for care, yet this flavor development is not simply outside flavor soaking inward. The better-supported mechanism is proteolysis inside the cheese. A useful comparison is bread crust: surface flavor can matter, but here the long change is largely internal.