Agreed, the type of person who can say "I'll just build my own CMS" is not usually the type of person spending a significant portion of their job time using a CMS.

And you might say, well if they're somewhat technical (which is much more likely, think about eg technical writers or product managers or marketing teams) they can use AI to add more features. But when you actually have something at stake security-wise, that means you need to either put them on rails with something much more prescriptive (a "trad-CMS" lmao) or spend a bunch of time reviewing/fixing their code (which, since they're not the same kind of person as you, may not even be something they have any interest in doing, and kind of just gets in the way of them getting their message out on your site as intended).

That said, I think most tech companies will still roll their own internal tools to do this rather than buy it off the shelf, just because buying it through a vendor and fully setting up in a way that's secure and integrated with your business processes involves more work than rolling it yourself, and has a lot of ways it can go wrong.

IMO what you really want is some kind of FOSS CMS that works really well off-the-shelf for a small team, and has a strong ecosystem of integrations to add on SSO and visual editors and stuff like that as you grow, where you can also probably just hire someone to do that part since that would probably coincide with your business getting too busy for spending your time on an internal CMS to be the most effective use of your time. Which is literally wordpress.

It's just that wordpress is a death-by-a-thousand-cuts of mediocre quality/over-complicated stuff, and the core technology has some bad abstractions/shows its age, and that emanates out into everything else it touches. Also, while it's true that a static site is much better for most people, SOMEONE has to actually run a web-server for those files, and that does actually cost money to provide, so I've softened my thoughts on Wordpress doing that. It's not actually free for Cloudflare to do that for you, it's just a loss leader they can afford to give away because they have economies of scale and privileged access to the Internet.