The whole point of the OGL was to encourage code re-use, make the D20 system "the" system for playing in every kind of game and genre, and mandate sharing of material derived from open content. The current GSL is, basically, an old-fashioned 'license to print supplements', ala the old Judges Guild stuff. It will not surprise me if it is non-viral, as well -- Necromancer's derived material will not be open to, say, Mongoose, and vice-versa.
A game system is not "code" to be "re-used". It's not the GPL.
d20 can do a lot, but I don't think it can do everything. The ones that "stretch" the system aren't really d20, can you really mix M&M, Spycraft, Arcana Unearthed, and D&D together without SOME conversion?
"Mandate sharing of material"--how much of that was really done? I think most publishers did their own thing, and didn't make a lot of cross-compatible products. The virus was mostly D&D to their own campaign setting or variant PHB. WoTC stopped creating OGL stuff outside of a few books.
Honestly, it was a great experiment. But I think it was a failure. WoTC primary market is selling "core books". The PHB has always sold more than the supplements. I think it was mentioned while Millions PHBs were sold, only 300,000 of the FR 3e Campaign Setting were sold. This has always been true--Gary Gygax once mentioned millions of D&D players but only 100,000 subscribed to Dragon magazine. Having the rules on-line may have hurt sales. I even suggested maybe the best thing to do with Unearthed Arcana would be not to convert it to on-line format, since I thought that the best way to reward WoTC for doing that would be to buy the book and show that you'd publish it.
No other major publisher has adopted a similar method for their created game systems--White Wolf hasn't opened the Storyteller System, Steve Jackson hasn't opened GURPS, etc. If the concept of open gaming is so great, wouldn't other publishers have followed suit?
And for those that still believe in the OGL, continue using the third game! Honestly, the people who are complaining the most are those who wanted 4e open. Wizards has said "no, it's not". So, if your primary goal is to support open gaming, using 3e INSTEAD OF 4e. It'll keep at least one thing I liked about the OGL--keeping a form of D&D alive if the parent company died.
Trying to "reverse engineer" 4e is just going to prove some hypocrisies. It's Wizards choice to use a different license system. I don't think there would be an ethical way to "reverse engineer" it. Granted, you might be able to legally do this. However, I'd personally take a dim view of a publisher that decided to do that. Wizards has their own license which (hopefully) allow you to keep your own product identity, so using anything other than the GSL would no be required. And unlike OSRIC, you're not trying to support a "dead" game with no license.
If you're trying to do something "completely different", why even bother making it 4e compatible? The question is, since (a) Wizards is offering a license and (b) those using the existing OGL can still use the 3e game, why would somebody like Green Ronin decide to upgrade M&M to be 4e compatible.
Honestly, what I think will happen is that instead of trying to make 4e compatible cames, you'll see more game companies come up with truly original ideas and not try to make many minor variations of the same ruleset. Those that make major changes could just as well create their own new games. Why make a knockoff of D&D when you can create your own game? With the d20 glut, I have a feeling people would rather get a totally new thing than another umpteeth edition of D&D.
I honestly think this is a good thing, the market had much more creativity before the d20 animal, and we lost some of that "bio-diversity" when everybody rushed into the d20 market. Now, we'll have those people who are cool with the more restrictive license who want to suppliement WoTC products or produce their own settings. And, we'll see more creative games.
Of course, it should go without saying that all Open Content published under the OGL was by definition derived from the 3E SRD, which itself falls under the Open Gaming License. It seems to me that such content would, in effect, have to be "re-derived" from the new SRD to be able to be published under the Game System License.
Technically, D&D is not derived from the SRD, the SRD is derived from D&D. D&D itself isn't subject to the OGL.
But even if so, I believe one of the major reasons for changing the whole "fluff" or background is to make elements of D&D require WoTC. The new planes, world like the Feywild and Shadowfell, new creatures, the new cosmology, etc. The key thing is to make the new elements so integrated with the game and so desirable that you'll want to use the GSL. I'll bet the GSL allows you to use all the "fluff" in an adventure or supplement, while the OGL will not.