Can't exactly branch out if I rolled a mage a decade ago and feel too close to it by now to reroll. I'd be happy to heal or tank an occasional 5 man but, eh, can't. Alts wouldn't solve the problem even if the expansion was more alt friendly unless somehow they made daily heroic dungeon rewards and mythic dungeon loot account wide as I'm going to do those on my mage anyway.
The fundamental problem with MMO design of course is that games tend to commit to a model where people first go through what's essentially a single player game and only after that end up in content where tanking and healing roles become significant. After that experience most players are already attached to a specialization as you wouldn't go through the weeks of leveling to max if you didn't like your pick and that's likely to be a DPS specialization. This is made worse by the existence of pure DPS classes as a lot of us got too used to our class long before we were even able to try the other roles (and back in vanilla it took a hell of a long time to level an alt to even try the other roles at high level).
WoW has somewhat of an excuse as it's an old game but it baffles me how new MMOs keep making the same mistakes, not guiding players towards group content earlier, still including pure DPS classes etc.