This isn't unreasonable, although I would say the idea of QB's "carrying" teams is a little exaggerated. Obviously some can compensate for more than others can, but the supply of Mahomes/Rodgers/Brady type players is very low.
That said, while I don't disagree with the idea of wanting the more cap friendly player, there are more details that have to be factored in. For example …
1. What would the Cowboys have to trade to get Stafford?
2. After weighing cap differential and cost of the trade, is it worth it to go with the player that is almost 6 years older?
3. Are there other cap saving maneuvers the team can make?