While running an errand I though of something that I was groping toward in the last post. In real estate leases there is the concept of "quiet possession." The landlord retains many rights, including the right to enter upon the property, to make necessary repairs and upgrades, etc., but there are limits if his activity impinges upon the tenant's legal right to "quiet possession" under the lease. "Possession" is ordinarily construed to mean use of the premises for the intended purpose.
The landlord also has certain obligations, also defined by law in most places. In general they can be described as keeping the place in good repair for the intended use.
It seems to me that "quiet possession" of leased property is analogous to the idea that, even in the context of grossly one-sided license agreements, users have a right and reasonable expectation to use the software they licensed on a one-time-payment basis, without being confronted with changes imposed by the licensor that force them to pay again to "upgrade" to the latest version.
Of less importance for our current discussion, the licensor may have an obligation to repair its own mistakes in developing the software (the "fitness for purpose" concept in manufacturer liability), though user agreements try to minimize it. Certainly, any overt action by the licensor to disable the software or otherwise make it inconvenient to use would, in a real estate context, be viewed as an outrageous attempt to break the lease and force the tenant out.
Of course, if Microsoft were to stop providing security fixes, it could in effect let the malware people kill off old software, unless the marketplace generated entrepreneurial solutions for that.