It is replacable, but the cost of doing so is pretty significant in a small start up.
The only real business benefit that other frameworks provide is that they make it easier to hire for. If things continue as they have been in a year or two we will have a bunch of new cool frameworks, and current super stars will become old. So why bother switching yet?
For now Angular 1 is not broken (assuming known limitations are OK for businesss need) so why fix it?
Aside from our own development we also purchase around 10-25 IT systems a year. Some of these come from minor firms, sometime we even buy development directly from student startups where we do a lot of the usecasing and teach them on real world application, and we wouldn't hire someone doing Angular 1.
That's just us of course.
We also have around 350 different (minor to major) systems running, which won't be upgraded in a foreseeable future.
Using old technology is a consequence of reality, but that doesn't mean it's a good idea. It's very dangerous to get complacent, and I think especially so, with something so replaceable as Angular 1.