> Proprietary software needs a stable ABI.

Open source software also needs a stable ABI because:

a) i don't want to bother building it over and over (not everything is in my distro's repository, a ton of software has a stupid building process and not every new version is always better than the old versions)

b) a stable ABI implies a stable API and even if you have the source, it is a massive PITA to have to fix whatever stuff the program's dependencies broke to get it running, especially if you're not the developer who made it in the first place

c) as an extension to "b", a stable API also means more widely spread information/knowledge about it (people wont have to waste time learning how to do the same tasks in a slightly different way using a different API), thus much easier for people to contribute to software that use that API