It's missing the point of a platform team. Regardless of how you choose to organize your larger business, each individual product needs similar things. Whatever those may be. Code forge. Binary artifact repositories. Build, test, and deployment pipelines. Development and staging environments. Workstations with a network connection to your business systems. You can do pay for SaaS enterprise stuff, build it yourself team by team, or have a platform team. The point at which it makes economic sense to have a platform team isn't when they can directly save money by saving time. It's when you have enough product teams that having them all devote effort to doing the same thing in parallel when it can be centralized is counterproductive, and your needs are sufficiently niche or the seat licensing is high enough that SaaS solutions no longer work, either.

It's effectively a form of administrative overhead. The question isn't whether the cost is justified. You have to do it. The question is whether you're better off centralizing the function in house or not. You see a similar dynamic with any other administrative function. Should you hire a payroll company or have your own payroll department? The difference is software developers tend to believe, and in many cases might even be right, that they can individually role their own platform functions, introducing a secondary consideration, whereas the idea of having software developers simply handling their own payroll clearly makes no sense.

Of course, it's worth another of the more obvious reasons it doesn't make sense. If you give your engineers direct access to the company bank account, there's a pretty clear risk they're going to steal from you. Similarly, there's a non-economic argument to be made for having a platform team in terms of separation of duties for security. Every business decision isn't simply made by answering what does this cost and how much will it earn me? Try running a software business where the developers are responsible for physical site security and see how long you make it before getting robbed. Try not hiring any janitors, letting your developers clean their own restrooms, and see how long you retain any staff.