I think Tua would redshirt if there were any other viable options. There aren't, and why that is, is almost another discussion. But as has been said, there's no one else to come in when there is a blowout or something. It would be insane to play Hurts every single down, the additional injury risk alone is too great. So, unless something dramatically changes, Tua will play.
I do think some are reading too much into this though. Tua is in this position in large part because no one else is there. Otherwise, AJ redshirted (though it was kind of close), Phillip Sims redshirted, David Cornwell redshirted, Cooper Bateman redshirted, Blake Barnett redshirted, etc... Hurts played because he was the best guy for the job, but AJ specifically did not play when he was the best man for the job. Tua will have to play, but it won't be simply because he's the best backup, but also because there's only one other scholarship quarterback.
Also, I don't buy the leave early hype. The quarterback position is complicated, not a whole lot leave early. It's still far more likely that the 5th year would have been of use than not. I've seen more 5 star QBs end up not having a pro career than I recall leaving early.