Addressing the "It's too late" Argument:
Not only is that a stupid excuse (Mojang should have prepared for this as soon as they knew what they were going to do in 1.8) but has nothing to do with the solution presented by the OP...
Separating files into older and newer version does not require a "1.7.11".
If you wanted to warn people not to open their worlds in a newer version, that's impossible, but if they locked maps to only the version they were made in, you could do that in a new patch; separate the saves into a folder for each update, and the ones that don't have a time stamp go in a different folder. Then you have to manually change which version you can open a map in, making it nearly impossible to make that mistake. it doesnt take that much imagination to come up with an idea to fix this.
It may seem over complicated, but is it really worth losing thousands of hours of work because the devs were too lazy to make a fix?
I don't think so.