Since the code is free you can just buy support from any IT company who offers it. You are not forced to buy it from the original manufakturer. So with Linux - you can basically get eternal support if you want it.
The truth is if Microsoft sold it off they could probably get very good payment from other companies that would love to take over support and upgrades of Windows XP.
Microsoft is killing the business to be able to force the customers to downgrade to their new operating systems.