Even with persistent comments and rumors, Microsoft has categorically stated, in no uncertain terms, that Windows 10 will not turn into a subscription model.
Of course, that may change in years to come, but as of today, no.
Your Windows 10, whether upgraded from a current 7 or 8, or bought retail, will continue to be upgraded, 'forever' for free.
Does this include an eventual Windows 11 or 12? Who knows.
The free upgrade to 10 from an existing 7 or 8 is only applicable until July 29 2016. If you want the free upgrade, do it before then. Otherwise, buy it.
Absolutely unknown if a current retail Win 8.1, then upgraded to Win 10, will allow that particular Win 10 license to be used on a whole different PC.
I've not seen any comments from MS on yea or nay for that.
They have specifically said that you can reinstall on that particular PC again and again. But nothing, so far, about a whole different PC.
They have said OEM becomes OEM, Retail becomes Retail.
They have also emitted verbiage to the effect of "for the life of this device".
Let us wait and see what the actual licensing says, after it is actually released.