What they should have done is draft quarterbacks every 3 years in rounds 2 and 3, perhaps 4 if it's a steal.
This gives the team an acceptable level of talent, an inexpensive contract, legitimate competition, future contract leverage, a potential high trade value, and a player that knows the team and system. After 3 years, they could trade him or keep him as the backup.
Getting a legitimate free agent backup is expensive, he would have to learn the system, and he would have limited value. Anyway, there really isn't anyone available that would actually make a difference.
Drafting a QB in the 2nd or 3rd every 3 years isn't going to dilute their talent when you consider the quality of a lot of the guys drafted in those rounds.
As it stands, it's Dak or bust. If he goes down, season over.