They're still expanding the trust system, we can't be sure that, in the future, they won't expand it to roulette to assure roulette to proc in a acceptable period of waiting time if healers are too scarce. Although I really doubt they will.
I'm only saying if a role become too scare in the future, I'm almost certain SE will use the trust system to fix the issue.
And as Mimilu said, if sprouts don't tag to standard dungeons and go through trust instead, there won't be any need for a lvl 90 player to go into Sastasha.
Finally, what will happen if, one day, the already instant queue trust system allows you to complete a dungeon faster than with pickups?