WotC didn't give them a reason to switch, didn't encourage and motivate people enough to try their game. They just assume everyone would play and switch because it was D&D. They've said as much in interviews the last few years.
That's on them and no one else.
So, really, the best lesson is that WotC might not have been the best stewards of the hobby...
And THIS is the true bitter pill for 3e/3.5 players. They created the doomsday machine.
When 3e came out, the brilliant plan was to kill off AD&D and D&D and replace it with D20. How to do this...simple...don't print anymore material for the older editions. Make it so that hardcopy is extinct.
Basically, if players want to play D&D, they HAVE to switch to the new edition. It was said, it was done, so it was history.
It was highly successful. There was NO reason, or so WotC thought, that they could not replicate this exact feat once again. They basically did the same thing to 3.5 as they did to 2e/D&D/AD&D. Exact same playbook, exact same plays. If you look at the early marketing campaigns for 3e, it's remarkable how closely they mirror 4e's marketing.
If you call 4e's marketing arrogant, 3e's was just as arrogant.
There was one difference though, and you can blame Ryan Dancey for that. The seeds of what occurred with 4e started all the way back prior to 3e's release, and that was when the idea of an OGL was created.
What 4e's release had to contend with that 3e did not, was an OGL. That's what made PF possible, that's what made other systems possible, and that's why what worked for 3e's release did NOT work for 4e's release.
IF T$R had release and OGL in regards to AD&D, it's actually quite possible you'd have seen a similar scenario happen with 3e's release.
I point this out to say, I can actually see the idea where if PF was not created, 4e may have been FAR more successful. I won't say it would have been, but at the same time, I'd say there's precedence for it.
Personally, I'm wildly glad at this point for the OGL and Dancey's idea. Why? Because I absolutely love PF and what Paizo has done.
But, the problem with many people is while they complain about how 4e came out, they are blind to their own actions and how 3e was put out, and how that was the pattern that was created for 4e's release as well.
It's basically a...you reap what you sow...type scenario...both for the Players of D&D/PF and for WotC.