Well, perhaps it already has but it's interesting seeing it pop up in real life from time to time.
In the past few days, I remember seeing it styling the backend calendar system for the gym that I go to and also powering the patient search when I went to get a blood test.
These are only two anecdotal instances of course but I wonder just how many systems are floating around the place that are plowing through work with that instantly recognisable basic Bootstrap template.
My assumption is that it has continued to succeed because it still primarily caters to getting started as quickly as possible.
If you go to the homepage, the two links for CSS and JS are right there for use straight out of the box. Similarly, they still distribute single static files too.
It feels like most CSS frameworks are only accessible via npm with CDN-hosted stylesheets going the way of the dodo so it makes sense that people who choose bootstrap would stick with it. Lack of complexity is effectively a feature after all!
Just imagine how much more productive the planet would be if we just doubled down on Bootstrap and threw away all of the other frontend tooling.