There is an example of companies that do just this: video game copanies. Valve, for example, still sells Half Life on the shelves and through digital distribution, even though there's Half Life 2 and newer games. They license the engine and allow professional mods using the engine.
It makes a lot of sense for PnP RPGs, in fact, because ultimately what is an RPG but a game engine ready for your modding?
Comparing video games and RPG's seems like an apt comparison to me.
Video games often come out for multiple consoles/operating systems: e.g., for Xbox 360 and PlayStation 3, with a later version coming out for PC. Or PC games sometimes eventually come out for Mac if they are highly successful. And PC games inevitably run for many different flavors of Windows, like XP, Vista, or 2000.
I believe how the video game console maker gets their money is licensing fees from 3rd party game companies: like $1 per copy of a game made for its console. They also make some first party games for their own system, to try to make a killer app that makes their console special (e.g., Halo).
Some of this went on in the d20/OGL era: I have a dual stat'd Harn/3e game, and there were Troll Lords games published for d20/C&C.
What's interesting with the 4e GSL is that WotC is saying you can't make a game that can run on other "operating systems". I dunno if it's only anti-3e, or anti-all other game systems, but if it's the latter -- "if you want to work with the big dog with the dominant market share, you can't work with the little guys like Pathfinder, C&C, etc." that's really uncool and fishy.
The other interesting software analogy is backwards compatibility. I've seen a demo that Windows Vista can run Visicalc, an ancient DOS spreadsheet program (Diaglo may use it for character sheets!). And I know Apple OS X has a built-in emulator to run OS 9 (and much earlier) applications, even though the chip and the OS are completely different now.
Obviously, that's more complicated with books, but still . . . some effort at backwards compatibility would have been nice. The official WotC view seems to be: "toss out your old campaign, your old characters, and all your old books -- you need to upgrade or get out, because our new ideas are so great that everything old is useless".
The car analogy doesn't work for me as well . . .