That's the thing. Technically, we already have. In the theoretical worst case scenario: MS buys Mojang. Decides to stop/disable modding, decides to be a bag of dicks about it, etc; that doesn't END everything. We all have licenses to use Minecraft as released by Mojang, access to future updates to Minecraft under MS could be held, but we still have every right to play with the version(s) we had prior to the sale. The downside being that nobody but Mojang (and MS by extension) has the right to distribute those files. Furthermore, under fair use we have the right to do whatever we want, including decompiling, to those files. Again, we just can't distribute those files. Mods would still be able to be written, and distributed, as long as they contained no code created by Mojan (which is legally true now (pre-buyout) as well). I don't know about the core files of forge, so I can't comment on how those would be distributed.
The final thing in our favor is how Minecraft functions. Minecraft is a "roaming" app. It's files don't "install" like a lot of programs do, making lots of registry changes and other crud in the process. So once you have a working "installation" of Minecraft, you can easily back up those pristine files to your choice of backup (presuming again that this is all done privately. otherwise that would fall under distribution). So in essence we don't have to worry about the older versions being locked behind a "always-online" gate.
And just to be clear, I'm not advocating any piracy. Distribution of any Mojang (or Microsoft) created content is and will be illegal. However we can hold onto our own files (that we paid for) as long as we want.