AbdulAlhazred
Legend
Extend it, yes, but still not infinitely. You can create a new edition and continue the subscription model into the new edition. You simply can't beat the hype that happens when a new edition is announced (some negative of course but by and large mostly positive).
It remains to be seen. I think they're going to give it a good try. Also I'm not sure they HAVE to have a system that is constantly being pumped. The beauty of a subscription based system is that as long as you have that decent sized core fanbase subscribing you can just keep making money. It might not be a mint, but its also very low risk and predictable. Keep putting out decent adventures and another supplement now and then and the game will keep cooking along. They can always produce other sorts of material like a 4e based MMORPG, CRPGs, possibly other things. They can also license out the system for other people's IP and have a revenue stream in DDI subscriptions for those as well.
Really the reasons there were edition rollovers before also had a lot to do with the mechanical weaknesses of previous editions. 1e and 2e were terribly limited and hard to customize to different settings and genre (and when they tried to extend 2e the system predictably pretty much fell apart). 3.x suffered from some of the same problems plus others. It did work pretty well in many respects but it also eventually collapsed under the weight of its own shortcomings. 4e seems to have been intentionally designed with a heavy focus on avoiding that kind of obsolescence. It certainly isn't the be-all and end-all of RPGs, but I think it finally gives D&D a core set of rules that can stand the test of time.
Heck just the fact that tweaking monster guidelines has such a great impact on play shows us that the game is capable of a lot of adaptation.