> asking an LLM to reverse engineer and make your own plugin is trivial.

If you already have engineers on staff, a few tens (or even hundreds) of dollars per month per plugin is likely a rounding error budget-wise. If you don't have your own engineers, you're probably not going to be able to produce something as good (reliable, well thought out, etc) as a commercial offering.

I had the same gut reaction as you, but the reality is much more subtle. We work with several clients who are bought into at least one of these ecosystems, and there's no way the math ever works out in favor of building an in-house solution.