So what if Windows gets a $5/m subscription option, and monthly updates that indefinitely allow you to update as long as you're subscribed?
Now this is a difficult one. Perhaps something like "beyond original service costs", so if you originally charged $5m / year you can continue doing so, but if you originally provided them free (and that was the deal under which your customers were persuaded to part with their money) then that is what you have to continue providing, unless you open source it.
What does "stand by its performance" even mean? Microsoft doesn't misrepresent Windows. Sure, it has bugs, as does every software.
A Ford Fiesta isn't the safest car in the world, but every time someone gets injured in a Fiesta collision they can't sue Ford for making the Fiesta less safe than a Tesla Model S.
Should we force Ford to take on more liability unless they open source all design and engineering documentation for their cars?
This is the point. There are standards that cars have to meet, and if they do not meet them you can sue the manufacturer. This is the same for ever other industry, except software (and religion). These standards would not be "no bugs ever", but would be more than the "no liability at all, run at your own risk" that is the current standard. Should we allow ford to take no liability for the functioning or safety of their cars? If not, why should we allow microsoft to take no liability for the functioning or safety of their software?
I would guess Microsoft would be more concerned with someone making a non-open source direct competitor. Considering all the things Microsoft is accused (probably rightly) of ripping off in the process of making Windows it is reasonable to expect them to not volunteer anything to the great unseen competitor that may well be out there somewhere.
I am guessing that the open source licence under which it would be released would be copy left, so it could not be used by someone making a non-open source direct competitor.