>but once it's done, it runs forever without a hitch.
Yeah... no. If you're dealing with changing systems, you'll need continued support from maintainers. And there's a lot of stuff out there in the business world that is commonly used and breaks all the time. Stuff will break. If not, it is not getting updated. In that case I'd be more worried about security than compatibility.
Yeah... yes. There are systems which are continuously maintained but don't break all the time. Yes, stuff will break but this is way less common in Linux.
Claws-mail has all my email for over 15 years. My inbox is several gigabytes in size, which claws handles flawlessly. And the software is continuously maintained. I'm using version 4.4.0 now, which was released 16 days ago on March 9.
So... yes.