If you think that's the only way an rpg publisher like WOTC can survive and thrive, then 4E is inevitable. As is 5E in a few years, and 6E a few years after that, ad infinitum.
I think that not only is that not the only business model, I think it's a short-sighted and ultimately ruinous one. But I understand that if you think the Rent Model is the only possible one, you've got nowhere to go but "Edition +1" as soon as you publish all the rules you can realistically publish.
I agree that it's not the only business model. But calling something that worked for 30 years shortsighted is a bit odd. I mean, the business of RPG's is only as old as D&D.
Also, I think you are missing a very important thing that WOtC has changed in their business model. The DDI. The DDI is what will keep WotC alive and well through all the edition changes (IMO).
Because with every edition change, they lose players. Or rather, they lose customers. Because they are who they are, they also bring a lot of new players into the business, but my guess, and this is just a guess, is that with every edition, fewer and fewer people are playing the current edition.
Now, with the DDI, WotC has just struck gold. When they announce 5e, a good chunk of the DDI subscribers will howl and demand Bill's head on a platter, wow'ing that they will never play 5e, because it's too anime and like a videogame. They will stick to their good old 4e. And thus keep paying for the DDI - because let's face it, the CB and MM alone are more than worth the $5-8 a month. Even if some people aren't convinced of that atm, I can assure you that the number of subscribers to the DDI will only go one way as more is released for 4e - up.
So, anyway. WotC releases 5e, but unlike with the previous editions, they keep a decent income from it, via the DDI. Because at this point, it's just money in the bank, since it's just maintenance.
The other possibility is that they keep a staff of a couple of guys that keep producing choice 4e stuff, released solely via the DDI.
/shrug