I think i see what you're trying to say, but i would not compare it to DLC and online activation because you have the option to buy phyiscal PSP games. The physical PSP games wont go away, you can play them in 20 years if you like, no problem. Here you have a choice, to buy physical games or digitial download games, this is not the case with games with heavy DRM.
To avoid paying twice, the only way to avoid this is by including a voucher with the game etc. to begine with. Since this wasnt done with the PSP games, it is not something that can be retrofitted. All PSP games are identical as far as i know, so there isnt a system that can allows a copy of a game to be registered one time only. I guess maybe your thinking more in general about this, but i just wanted to point out that such a system cant be retrofitted to work on PSP games.
Regarding Vita being backward compatible, i actually think this is somewhat debateable. I mean, the PS Vita can play PSP games indeed, so i wont argue much against it, but it is basically just a PSP emulator. Is the Wii concidered as being backward compatible with the NES, SNES and N64? Personally i would not concider that as backward compability. And are people angry on Nintendo for not releasing a NES, SNES and N64 addon for the Wii that lets you play the physical games? Because here you also have to rebuy the old retrogames. Now, one can argue that these games are old, so it make sense, and that is true. But i still think that the principle is somewhat the same, that the console supports old games, but only through digital download.
Regarding DivX, i remember well when it came out, but this is more of a discussion about Digital Distribution VS Physical Media. That is another discussion, it doesnt have much to do with lack of backward compabilitly in my opinion. But about this other discussion, there i support physical media the most because i like to collect games