We don't know yet, but we do know that Romo's injury in 2015 prompted the team to finally get serious about spending a draft resource on a QB. Remember they tried to move back into the first round to select Paxton Lynch?
I don't think they had visions of Dak being the future QB, but I do think he was drafted to solidify the back up position because they haven't been serious about filling that position in a few years.
When Romo was hurt yet again, I was ready to roll with Dak. If he tanked then we get a high draft pick and I didn't want Romo coming in trying to save the day with another 8-8 record wasting draft capital. If he worked out, then great. Either way, Romo needed to be gone.
As for who made the decision, we don't know that either, but I think it is more likely Jason made it because Romo has a strained relationship with Garrett when they used to be buddy-buddy, but still has a good relationship with Jerry. Doesn't make sense to me Garrett was advocating for Romo and that somehow upset Romo, but he is fine with the person that supposedly gave up on him. But, I could be wrong.