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#941
MS would have a problem if they did turn it off.
Under the terms of the EULA, a retail key can be used an unlimited number of times as we know.
Also the EULA clearly says upgrades inherit the characteristics of original key.
As you do not get an upgrade key from 7/8, only a generic key and digital licence, then you only have limited transferability using the activation troubleshooter.
So by turning off activation using old keys, the windows 10 upgrade would be downgraded in status in effect to oem (but with some limited retail status).
MS would not be complying with their own EULA!
However, MS could change things so only retail keys worked in the future.
I do not think MS really care that much though - they want as many people on W10 as possible.
They were prepared to give away 1billion upgrade free. By allowing upgrades to still be done unofficially, they edge towards that goal.