There are probably still a bunch of us that were fans of D&D and CT/MT and aren't playing T20 or 3e. If you've got a lot of time invested in development of older game-compatible materials, and your choice is spend your more-precious time developing them into new rules systems or just playing under the old system, you tend to be happy with existing rules - they're familiar too. After all, rules are only a vehicle for stories, so any decent ruleset will suffice (change as required on the fly).
I find from my groups that the only adoption factor for 3.5e D&D and T20 is the unavailability of the older books (and new players who either haven't played RPGs in many years and either never had a book collection or got rid of it). We decide to play an RPG, and everyone has to have some sort of basic books, so we have to pick what's going. In the T20 case, I have MT and it has been the kind of game (thanks to the UTP/Task System) where you don't even need a rulebook to run it that, as a consequence, the players don't buy. So the only T20 adoption has been me making a concsious decision to buy T20 and T20-suitably-generic products to support Hunter and Traveller to keep the game going. It isn't that I plan to use the ruleset. I will use generic enough modules and playing aides (deck plans, etc).
There comes a point where you've probably played 30 or 40 RPG rulesets. At that point, you realize none are perfect. Good enough is literally good enough. So you find a set you like, settle into it, and focus on stories and characters, which can usually be built fine in any ruleset.
I'll continue to support T20 because I like the universe and the user community, but I'm not moving into playing it for the ruleset. Sadly, the best ruleset (IMO, no offence intended) is still MT.
And I'm 35 now, if that matters. Started in RPGs in 1980. Traveller was my second RPG after AD&D and has remained so (sometimes momentarily leaping into first).
