Pheace: If you're going to consider 20 years as not viable for a 'rental' model then you should probably apply that to the 'GOG model' as well. Look back 20 years, see where gaming was then and how much you still use that, or are even able to use it. In 20 years, I don't have any particular faith that I'll still be using/able to use my GOG games. I don't even know if I'll be using a PC anymore.
Interestingly enough, the fact that we "only" buy licenses is actually beneficial for us in the long run. Because due to owning licences and not the games itself, every "refurbishment" done on the games by either the licence giver (eg. EA) or our "subscription partners" (GOG/Steam). Pretty much every time one of those make an old game runable on modern hardware, you can get the patch yourself for free.
Recent legislation even suggests that purely digital delivery actually might have more rights as retail, because due to the physicality of retail, a game might be seen to be sold as "as is" and no longer viable to non-critical updates. Because in 99% of all cases you always have a "service contract" with either the seller or the party who provides the service (eg. Steam), therefore the "sold as is" can't fly.
So keep those E-Mails backed up!