In my last job we ran centralized clusters for all teams. They got X namespaces for their applications, and we made sure they could connect to the databases (handled by another team, though there were discussion of moving them onto dedicated clusters). We had basic configuration setup for them and offered "internal consultants" to help them onboard. We handled maintenance, upgrades and if needed migrations between clusters.

We did not have a cluster just for a single application (with some exceptions because those applications were incredibly massive in pod numbers) and/or had patterns that required custom handling and pre-emptive autoscaling (which we wrote code for!).

Why are so many companies running a cluster for each application? That's madness.