Usually all it takes to keep one's software running on macOS is compiling against the latest SDK once every ~5 years. Depending on the nature of the app, source changes often aren't even required. As a dev I don't think that's too much to ask.
A lot of that is going to depend upon perspective.
Take something that is coming up: Apple has mentioned that Intel application support is going to end in the next release, or about 8 years after the introduction of the M1. Eight years may sound like a good run, but there are all sorts of edge cases: software that doesn't receive updates because the publisher is no longer in business, or software developers who are providing updates under a disagreeably different business model (ahem, subscriptions). Some software may have been released Intel only after the release of the M1, simply because the developer wasn't going to test a new architecture immediately after its release.
I'm not going to pretend that I know how much software that affects, because there was a 12 or 13 year gap in my use of Macintosh. Something I did notice after my return to macOS was the absence of software due to earlier changes in the platform. An more exotic example is F-Script[1]. Not only is the project gone, but the change in the security model pretty much ensures that nothing like it will ever exist again. A more common example will be games, where only a select few will receive updates a couple of years after its release.