We're going to wrap our current 3.5 campaign. Given that I'm the DM and had grown to hate the 3.5 system even before 4e was announced, that may not be too incredibly long. I won't end it just because I want to change systems, but I will say that the hope of converting the campaign to 4e was one of the things that kept the game alive after last year's announcement.
The biggest issue is that some characters (gnome druid, for one) just can't be adequately converted. Strangely, it's some things that were originally viewed as baggage (arcane spells for the elven swashbuckler hitting a PrC, the wizard's wrack spell, or the druid's shape-shifting, for example) and have now become iconic to the characters that are keeping us from converting. Had the campaign started in 4e, the players would probably have been able to build their characters more to their original vision, and enjoyed it just as much. But, now, they are constrained by design differences between the editions.
That's not necessarily a sign of bad design for 4e. I would have had relatively little sympathy for the player who just loved to play elven fighter/mage/thief characters in 2e when he whined about the design of 3e cramping his style. Such is life in a class-based game. If you don't like it, go play Hero. D&D is not -- and never should be -- a classless game.
As far as 2e not having hold-outs.... If I remember right, 2e had so many die-hard fans that the company went bankrupt and was only purchased because someone with deep pockets wanted to do a new edition. And those deep pockets were originally filled by CCGs -- which was a sure sign, to some, that D&D was dead.