I'm not a fan of Tua. But putting my Texans hat on, taking him as part of the trade means I can give him a two year run out and see if he can develop while I use the picks I got for Watson to build out the rest of the roster. Of course, all of this is dependent upon the new head coach and his view of Tua, and he's not there yet. I can't believe that anything is going to happen with Watson until there's a head coach in place.
As the Texans, you can execute the same exact plan with Darnold instead of Tua. But in this scenario you have the 2 pick, which is more valuable than the 3 pick and ripe for a massive trade down haul.
Like I said, if I’m the Texans GM, this trade is not about Tua vs Darnold. It’s about the value of the 2 pick vs the 3 pick.
The 2 pick allows you to take your next franchise QB. The only other move here is a trade down and the 2 pick makes that more likely.