If you're using Photoshop daily and other products occasionally as well then, sure, $50/month might seem like a reasonable cost, and I presume that the bulk of Adobe's users are creative types who do use the software every day and presumably benefit from regular new features and innovation. However, anecdotally I know that I'm certainly not the only person who has always used Adobe products, but much more sporadically.
I'll typically use Photoshop fairly intensely for a week when I have a particular job to do, but then a couple of months can pass without it being used at all. We use InDesign a handful of times a year. I'd suggest that for this kind of user 4 years ISN'T an unusual upgrade cycle at all, and in fact it may be way longer. In fact, I'm still using an old copy of CS2 which I think dates back to the mid-2000s? I'm sure people will scoff at that, but for what we need here, it's fine. Without wishing to litigate all the alternatives, Photoshop CS2 still does things which (just for example) Paint.Net doesn't, and more to the point we have years-worth of client design work in Adobe formats. Our quandary is that during various office moves we've lost the installation CD and the PC it's currently installed on is ready for the knackers yard. The path of least resistance would be to stay with Adobe - we know our existing files will be fine, there's no re-learning time costs and we get all the applications we need in one go - but at $600/year (actually more for us, since we're in the UK) it kind of forces you look at alternatives like Affinity.