IMO, until MS gets it's Win 10 running on a billion PCs, they gonna always leave a method to get the free upgrade.
They WANT to be able to advertise that.
By the same token, they do not want to "look" desperate to reach that number. So, they make it weird to upgrade, but they still allow it.
OP can ofc, just do a clean install of Win 10 using his Win7/8.1 key, which he can "get" using AIDA64.
Either way, MS is not going to "force" ppl to pay for Win 10....yet. Once they get their 1 billion Win 10 installations...Win 10 will probably cost $300. LOL