On the contrary, the difference is precisely that an abandoned FOSS program can be picked up by someone else; that this ability is not always used is beside the point.
It rarely if ever is.
Anyone remember PhantomJS? It took years before a replacement emerged and nobody was willing to take in the project, and this was a critical piece of tech at the time
Maybe PhantomJS wasn't as critical as you thought it was? Or maybe the JS ecosystem doesn't lend itself towards this model because JS developers have the attention span of a goldfish
You can always pay someone to maintain it. Not every thing has to be free.
It happens all the time. But when it happens it's so transparent that nobody even notices.
There are many 30+ year old open source applications still in use. How many of the current maintainers are still the original developers?