In regards to your first question, Win 7 _is_ pretty great. Windows 8 is more the OS with the funky tiles and touch interface and 10 is the answer to those complaints. It has some of that still, but it's a nice hybrid of the features we saw in 7 and the features of 8. For me at least, 10 fixed all of the problems I had with 8.

For the 31 days thing, I strongly suspect it's mostly just a marketing thing. "Try the newest Windows out for 31 days, risk free!". I could see it being an issue of storage on tablet devices with minimal storage or laptop/desktops with similar issues.

I've since wiped out my Win10 machine (by accident...oops) and went back to 8, since that's what I had a license and disc for. Not sure if I'll go back to 10. The privacy issues concern me greatly.