For me, I don't have a problem with upgrading from Windows 7 or 8 nor will I be leaving the Insider Program. There will be some who will leave the Insider Program, I'm sure; however, there will be some like me who will want to stay on the Program.
I always do a full format when I change operating systems. That way I get to set my system up exactly as I want it. Old programs will be gone and new versions (of those programs that don't auto upgrade) will be added. There are always some things that inadvertently get into the Libraries instead of on my external hard drive. That will be taken care of.
I don't understand your meaning here. Historically, when upgrading from one OS to the next, we've bought an upgrade package (which is less expensive than full) and installed it on our computers. Once the upgrade package is installed, the original key (whether OEM or Retail) is no longer valid. With Windows 10, the "upgrade package" comes free, so why in the world wouldn't I give up the key to Windows 7 or 8.1 if I want to upgrade to Windows 10? As far as I can see, the EULA will be pretty much the same, except that once installed on a computer, the license is good for the supported life of that computer.
Lemme see now . . . I have a Retail license for Windows 8.1 and I want to upgrade to Windows 10. OK, I have choices here . . . either buy a full package, an upgrade package or use the free upgrade package. Hmmm, decisions, decisions, decisions . . . :)
Yes, I can see someone with a system that's over five years old not wanting to upgrade to Windows 10 because it might die in a year or two, but those users will most likely be few and far between. Further, if the computer craps out, they most likely won't have it fixed; they'll buy a new one that will probably cost less than the old one did. And, it will have Windows 10 on it, so . . .
Free Windows 10 for the Insiders? Probably not, but if it happens, Microsoft's public image will rise exponentially. Am I holding my breath? Not no, but heck no!