Your question/concern is also related to Windows Phone "codename Mango" and version numbers
For reference, "Windows Phone" was used as a term in some marketing and promotional material for devices running the Windows Mobile 6.X operating system.
From a marketing perspective "Windows Phone" is clearer than a generic, non-specific term ("mobile") that can be further confused by a variety of numbers. I suspect that was why it was used then. I also suspect that's why it's being considered for [re]use now. (To avoid confusion between 7 & 7.1|7.5.) Plus it wasn't widley used in relation to WM6.x.
Tagging is a folksonomy. If 1130 people say it should be one thing and 1 (or 44 - although I expect that most, if not all of them, follow both) say something else then the larger group win.
Tweets are generally less a lot permanent than questions on SO. The rules for tagging there don't necessarily apply here. The twitter hashtag change is a response to dealing with identifying 7 and 7.1|7.5 specific tags. The specific version isn't as important on Twitter and so it is appropriate there. If and when Windows Phone 8 is released I suspect we'll see #wp8dev tags at that point to distinguish there.
Windows Phone 7 IS the product name. The expectation is that the product will be publically rebranded "Windows Phone" (without the 7) when "codename mango" is officially released. At that point the OS will still be "Windows Phone 7.X" and the development tools/SDK will still be "Windows Phone 7.X".
The 7 in "Windows Phone 7" was never a version number it was part of the name. The version number was "7.0".
windows-phone was created as a generic tag and has historically been used as such. Although not much and has mostly been used in combination with the
windows-phone-7 is generic to all minor versions of 7.x.
"windows-phone" is like a super generic tag. Especially when it's Windows Mobile heritage is considered.
It is WAY to early to make any assumptions about future version of Windows Phone  either major or minor. It's not realistic, therefore, to make assumptions about future proofing.
Retagging [general] questions so they are then tagged with a tag which is followed by many, many fewer people is likely to lead to those questions being less likely to be answered or answered well.
Mass retagging is not appropriate if a synonym is not also created. Without this people will continue to use what you deem to be the wrong tag. Unless someone can forever go and change these (and no one can) then we'll end up with the situation we now have with the 7.1 and 7.5 tags (see link above) where people continue to create questions which don't match the choice of the person with the motivation to go and manually reedit a load of tags.
There is no guarantee that devices running "Windows Phone 7" can be upgraded to any future version of "Windows Phone". The only guarantee I'm aware of having been made previously is that devices running "Windows Phone 7" will be able to be updated to future versions of "Windows Phone 7". There is no guarantee about future compatibility with "Windows Phone 8". I can no longer find a record of this anywhere but recall it from conversations held with staff at Microsoft when the platform was first launched.