There's a fair compromise...
That vendors are required to maintain support for software while an[1] operating instance remains in the world[2] ... OR until they publish the full source code - all needed to compile working instances - under a licence allowing study, support, distribution of altered versions, extension and patching, which in practice is going to be a GPL.
Then if the task is onerous and no profit can be made from it, the company loses nothing by publishing, and ends its responsibility. If it is a business decision, then their ex-customers get to make a business decision as well, and people who like supporting that sort of thing, likewise.
[1] You might put a number higher than 1 on that, or not.
[2] or country, or business, or public service, or government...
Biting the hand that feeds IT
