That's a great analogy. My only addition would be the nuance of that data modelling is way more flexible than authentication (and this is said as someone who is continually surprised by the business requirements, standards, and complexities of auth). Data modelling, after all, needs to handle the entirety of reality (at least what can be mapped to a computer). So you're more likely to outgrow it.

I've heard plenty of stories of folks moving from homegrown auth to a off-the-shelf solution, but that's because I'm in the off-the-shelf auth space.

It'd be super interesting to hear stories of folks who went the other way, and outgrew their service provider's auth.