he can, but as you said, much more valuable at 3rd.
DHs come and go, don't really need to invest heavily. Rooker looks like he's on the fast track, between him, Sano and Mauer you've got first locked down for the foreseeable future, you might have a half season to full season gap between Mauer and Rooker, but FA stop gaps are available at first.
I really want Sano to stay at 3rd, but what if the rigors wear him down quickly? In a couple years maybe?
There's not much in the minors for the hot corner. You don't really see 3b change hands a lot. Stop gap types, sure, but the Josh Donaldson deal from Oakland to a Toronto was unique and Oakland got fleeced. I doubt the Twins could replicate Toronto's move.
Rooker has been fine but he also hasn't set the world on fire. He doesn't seem like a 2018 guy, more of a mid-season 2019 guy. He's also profiling as a corner OF option and occasional 1B/DH. I think it'd be presumptuous to think he's the heir apparent at 1B.
Sano also looks fine at 3B. Not a gold glove but eminently capable.
Still think there's nothing but room for Mauer as the strong side of a 1B platoon through 2020.