You are right, people moved from tanking and healing to dps for RDM and SAM. New jobs aren't always the cause though. I dropped ast for a time in 3.1 simply because I was getting runs so painfully slow that the total run time was somehow faster for me to go BLM and wait a queue than go AST or WAR and get instantly. People change for all kinds of reasons.
Also, the dps doing different things doesn't mean there's more variety to a player. I HATE playing MCH and I can somewhat take Bard. I HATE playing SMN. Looking at healers, I don't like WHM. I like Diurnal AST, but I can't stand WHM's boring playstyle. I actually like how all 3 tanks play, but I hate playing them unless I'm in a party with people I know. It's not just about the playstyle.
Also, technically it happened twice. One time they released both a new tank and healer, as well as what's still apprently the least played dps (MCH) and after some time the queue's went back to normal. Same here, 2 brand new super hyped dps jobs, some time passes, queue's back to normal. They already saw what happened with DRK, which in case you forgot, already had it's niche of being the magical tank that made it get taken into raid over Paladin since Paladin had less magical mitigation than Dark Knight, plus less dps. So not only did the tank % not really change by the end of the day, but having just that type of uniqueness in mitigation caused Paladin to get pushed out. Kind of like how Samurai's are saying the same thing right now.
Also, BLM barely changed. They got Foul and a super swiftcast. Their rotation stayed roughly the same, just instead you fill the downtime with Foul instead of standing there doing nothing, and you get 2 more Fire IV's that I've seen BLM's complain about the existence of since it came out since your entire rotation circles around this button. Paladin gets to be a spellcaster and Warrior gets useful aoe utility.
If you WANT to give an idea of a brand new type of tank that isn't a dodge tank that makes or breaks the game, please help out. I always here dodge tank even though that'd destroy balance even moreso than it is right now.