It's a complicated conversation because the tools support many different ways of working - and most of them can be made to work with different tradeoffs.

A team that e.g owns its own microservice IaC can absolutely maintain a setup long term with app & db in the same state, it just requires care and love (it's actually easier, it just doesn't scale).

As you split things across more teams or have more complicated IaC "supply chains" (eg teams supplying modules to each other or have lots of ppl working on different bits of the same thing) you need to look at ways to make things more foolproof, easier to support and give yourself as much "wiggle room" as possible for upgrades. At this point having state split out is very helpful (almost essential).