Ultimately the Vikings opportunity to sign Cousins who they perceive to be a franchise QB wasn't going to be derailed by future contract negotiations. The NFL is dominated by QB play and when you have the opportunity to grab one you don't pass it up and figure out the rest later. Fortunately the Vikings have managed their contract situations extremely well and have the ability to really massage anyone into the 2019 cap.
When it comes to Barr, Diggs, Hunter, and possibly Richardson there are two sides to each negotiation. Although the Vikings may have the space and want to retain each of them it doesn't always work out that way. Other teams can offer funny money to pry a player away or a player could simply not want to remain in MN. Not saying that is the case but there are a lot of qualitative aspects to negotiations. How'd you feel if the Vikings passed on Cousins and Diggs decided to jump ship to pursue another opportunity with better QB play?
The key for MN is to lock up at least two of Barr, Diggs, and Hunter this off-season to avoid inverse tag leverage. The tag can be a good team negotiating tactic until you have more than 2 players you're fighting to resign. At that point each player can ask for more than the tag since at that point they can see themselves getting free and getting crazy money on the FA market.