This is a fair point. I fully understand wanting to keep Teheran. There is definitely a large part of me that would like to keep him and would worry about our pitching without him. Having said that, there's also part of me that worries if we keep him, he either regresses, gets injured, or both. Injuries are a risk for anyone but especially so for pitchers.
And the bottom line is, we are loaded with future pitching talent (NOT, I repeat, NOT a surplus! Do not read this as me thinking we have a surplus!) and are not quite so loaded in legitimate bats. So Teheran represents our best option currently to go get some legitimate bats. It would certainly hurt our staff, but that's what you have to do sometimes.
I'm ok with whatever we decide, assuming that if we trade him it's for a really good haul. I like Teheran and would love to have him anchor our rotation during the transition, and I would also be more than ok with adding some bats that can be ready fairly soon. It's not an easy decision, but it could propel the rebuild forward even further.