I have been a player and DM of D&D and AD&D since early first edition days. I started with the red boxed set with the painting of a warrior and maiden battling a big green dragon in a dungeon setting. My first adventure was B2 The Keep on the Borderlands. I played and ran AD&D from the time that it consisted of a Players Handbook, a Dungeon Masters Guide, a Monster Manual, and a Dieties and Demigods. The Fiend Folio had only just come out when I began playing and DMing the game. I saw 2nd Edition supplant first edition in 1989. I watched people complain about their favorite things in the 1st Edition game getting dropped from the ruleset. I followed the hype on 3rd Edition and ran my first 3rd Edition tournament without a good working knowledge of the game. And from all this observation, I can say that D&D 3.5 will not die by 4th Edition. 4th Edition might become the default supported edition of Wizards of the Coast, but it will not be the only edition being supported altogether.
When AD&D 2nd Edition supplanted 1st Edition, the rules differences weren't that noticeable. You could actually use 1st Edition materials to play, and no one would be the wiser. In fact, many people did use their 1st Edition materials with their 2nd Edition materials. I was one of them, as I chose to make the King Arthur's Britain game a hybrid using whatever rules that I found that I felt worked. I used the 1st Edition assassin class with the AD&D 2nd Edition ruleset until I decided that the AD&D 2nd Edition assassin kit worked better. My campaign rules continued to morph as I uncovered new rules that handled the paradigms I wanted to explore.
When D&D 3rd Edition came out, I was reluctant at first. I would not have played or DM 3rd Edition at all had I not already been involved with the RPGA, as I had just joined the RPGA to see what the organization had to offer. Because the RPGA was going forward with 3rd Edition, I had to get familiar with 3rd Edition. I often say I was dragged into 3rd Edition kicking and screaming because I didn't suddenly jump on the bandwagon with absolute enthusiasm. I resisted. In fact, I was still using AD&D 2nd Edition conventions to design monsters until the D&D 3.5 ruleset convinced me that 3.5 was the better way to go. Meanwhile, I allowed my players to choose which edition and game setting I would run, with the setting and edition closely tied together: Arthur would be run in AD&D 2nd Edition; Greyhawk would be run in 3rd Edition. My players were split down the middle, but ultimately it came down to Greyhawk in 3rd Edition.
In 2006, when I got my friend from Pennsylvania, Cullen, to start playing in my online D&D 3rd Edition game, he invited me to play in his Forgotten Realms game, run in 2nd Edition. And in the same year, an OGL ruleset for AD&D 1st Edition was released as OSRIC: Old School Reference and Index Compilation.
It must be put in context here that when AD&D 2nd Edition was released, many old-school D&D players dropped out of the scene or picked up other games, while many hybridized their games with 1st Edition material or played pure 1st Edition gaming. When 3rd Edition came back, the hype was so great that many of these players returned, and now with the coming of 4th Edition, there are people who use a variation of a Star Trek movie adage: "I only play odd-numbered editions."
While a vocal segment of the gaming population have lamented that 4th Edition will make 3.5 obsolete, I have to say that with the current gaming landscape, I think that this is impossible. I believe that every flavor of D&D and its variants will not only be supported, but it will thrive. Take a look at the variations of D&D that are currently available:
D&D 3.5 and its d20 derivatives
AD&D 2nd Edition
AD&D 1st Edition
Hackmaster 4th Edition
Add 4th Edition, the advancement of the OGL to it, and the continued background support of the 3.5 OGL, and you have a vast number of choices for your D&D-flavored roleplaying.
You have D&D 3.5 and the 3.5 d20 games, AD&D 1st Edition, AD&D 2nd Edition, Hackmaster 4th Edition, OSRIC, and any hybrid of any of the editions possible.
I believe that companies can continue to be viable using the D&D 3.5 ruleset rather than moving into the 4th Edition development. Companies are already making a profit supporting old-school gaming. And Wizards of the Coast will not make nearly the profit on 4th Edition that they made on 3rd Edition. Those days of publishing a new edition to make more money are over.