Stroman came up to pitch out of the pen, and did practically nothing for however long he was up (2 weeks?). They sent him back down to get stretched out again before calling him to start permanently. He was starting in AAA prior to that as well.
You can't just have someone pitch in the pen, and then immediately let them throw 100 pitches right out of the gate. It will take 2-3 starts to get their pitch counts to a level where they can sustain it going forward. If Estrada is in the pen, and then is needed to start, his first start will be something like 65 pitches (give or take), and then it will gradually go up after each start. They need depth that is already stretched out, whether it's Norris or someone else.
Norris starting in AAA will have way more benefits than not. Delay his service clock, keep him stretched out, a little more AAA seasoning, etc. If Norris absolutely dominates Spring Training, then maybe you consider it, but right now, if Sanchez is locked in for the pen, then Estrada is the defacto #5 starter. As he should be.