Teams with a good but not great QB are in an extremely difficult situation. Without a backup plan they are hosed in the short term, having to go in to the following season with somebody like Mike Glennon or Josh McCown, until they can find a long term solution.
The other alternative is to sign a guy you don't truly believe in to a long term deal with lots of guaranteed money, which makes it nearly impossible if you decide to move on a year or two into that contract.
Considering the money that Washington has shelled out to Cousins over the last two years, back-to-back franchise tags was even worse than either of those alternatives. If they had traded him a year ago they could have bottomed out in 2016 and had an early draft pick, and that new QB would by now have some experience.
This has been a smart business decision by Cousins. He makes a boat load of money in 2016 and 2017, and still gets an opportunity to really cash in as a free agent in 2018. Well played by a guy that isn't a top tier player at his position.