They’re not just writing the software, they’re responsible for the infrastructure it’s running on. And keeping that running and secure IS a full time job.
Right now, you sound exactly like one of those C level execs who looks at IT and asks “We haven’t had an issue in years, what do we need to pay them for?”
Even if you have a full-time role for continuously auditing the infrastructure (which I would say is the responsibility of either a security officer or a devops engineer), you still didn’t show how that needs a 15-person team, and an otherwise-untouched infrastructure should just keep on working (barring sabotage), unless someone really messed something up.
If CI builds or deployments keep randomly failing at your place, that’s not an inescapable reality, that’s just a symptom of bad software development practices.