Solo content dictates that going DPS is the best/fastest way to play, so you really can't blame players for getting comfortable doing those roles.

If a certain core element of your game can't even meet its demand after dangling virtual bait to lure players to do it, it might be time for modern day MMOs to realize the playstyle just isn't fun to play. Perhaps make it ACTUALLY viable in some way for a tank/healer to solo leveling content and not build the leveling experience from the ground up to cater to DPS roles, then maybe players will be more than willing to try out something new. No amount of virtual presents will ever make up for a the boring trek of yelling at monsters or sprinkling bath salts on for your team for a straight hour.