Odd that you'd blame WoW, which had a far softer approach to (greater hybridity in) roles than earlier MMOs and still a considerably softer/more flexible approach to role capacities in all but 10+ man raids, let alone immediately before going after XI and 1.0's, whose takes on roles were likewise much more flexible (with 1.0's "tanks" varying having very distinct places in party play between brawlers, vanguard/trappers, and squad leaders).
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That said, I agree wholly that roles should refer to what we do (which would be neither permanent nor mutually exclusive), rather than what we are, and jobs should be built first and foremost around job identity. I feel that matchmaking's requirements for X of each job type should be replaced with a minimum of X of given capacities, where one could as easily make up for all but burst mitigation with kiting (since, if given sufficient enmity a al Shadewalker or the like, kiting is still taking). And if we did that, a real BLU could walk right in.
NIN, BRD, MCH, SMN oddball speedrun groups? Bring em on. No fear double-tank dungeons, with tanks having more control over their mitigation (rather than sacrificing so much offensive throughput for absurd levels of passive mitigation)? Sure; I'd love to have actually nuanced tank-swaps rather than just those forced by Vulnerability debuffs. DRK, NIN, WHM, BLM for maximum shadow-dancing (each Flare or Holy giving opportunities for shadow users) and evasion? Hell yeah. But that required job identity and job identity requires both undermechanical depth (our content actually doing things, of interest, frequently) and a new approach to roles.
I don't think that'll happen, nor do I think we need to go so far to get a BST that's still miles more fun than almost any existing job, but I do think the game would be hugely improved in the long term by changing to that kind of design paradigm.