If you want to talk about rigid, look no further than me. Up until last month when my last Windows XP computer died, I was using Paint Shop Pro 3.11 for my image editing needs. To give you an idea of how old it is, I recall the second-last update (3.1, I think) was major because it allowed the use of longer-than-eight-character filenames - which, if you are old enough to recall, were introduced with Windows 95. Assuming it was at least a year old at that point, and I think it was, I was until recently still using an over-twenty-year-old program!
And for the most part, it worked well. Pretty much like MS Paint with a few other things. Other than transparencies, everything I needed to do was easily accomplished with it. Not that I couldn't do everything in GIMP and other current-or-at-least-more-recent-OS-compatible programs, but I knew where everything was, as was just comfortable, maybe too comfortable with it. I tried later versions of Paint Shop Pro, but they had an annoying anti-aliasing effect when copying and pasting within 16.7 million colour images.
I gave in to the fact that I shouldn't rely on a relic forever (well, by default), so I started using Paint Shop Pro X 7. Some things I've learned and familiarized myself with already, I guess I am still adaptable, but also already turned into one of those old farts that are stubborn about how things used to be and never felt like even attempting a change until now, where I have to (short of installing XP on a new machine which would be silly). Soon the trial will be up and I'll have to decide to buy it or use GIMP or find a trial for PhotoShop or something. Or maybe I should just go ahead and buy Paint Shop Pro X 7, considering that the Paint Shop Pro 3 I was using for two decades was an unpaid shareware version...