Extending support for people who actually paid for the licenses once is one thing, although of course that has to end eventually,
Why? People paid good money for working supported product. Just because Microsoft wants to bait and switch doesn't make it right. I hope some deep pockets corporation sues the bejesus out of them to force this issue.
because a one-time payment can't support a development team in perpetuity.
Oh, so that's your reason to allow them to bait and switch. Look, they should have charged a proper amount to cover whoever does support. It isn't the customer's fault that Microsoft didn't price their product accordingly.
Be that as it may, one solution to the problem you gave is to force Microsoft to release the source code to legal licensees if they are unwilling to support the product they sold. That way at least someone can support it for working hardware. But oh, that would stop the bait and switch... Can't have that!