The problem for Washington is that Cousins will want to be the highest paid QB in the NFL. They're going to have to be completely sold on him to do that because that contract means it's the last QB that the GM and coaching staff will choose one way or another. Cousins may have it or he may be the next guy that has one good year then everyone goes, "Oh, we should probably game plan for him" the next year and he's back to mediocre.
I'd also keep in mind that Washington has had a good year on the injury front. In terms of straight up man games lost, they are 18th in the league. They're also 3rd in their division(the same place as the Bills who have are 5th in terms on mangames lost at 142 with lesser QB play) so as amazing as Cousins has played this year, they'd be banking on him being *better* in the upcoming seasons as his contract would limit their ability to really upgrade the overall level of talent on the team and they'd need to remain relatively healthy.
I agree that if he plays as well as he has the rest of the season that they're unlikely to let him walk and will get him signed, but I don't think it's a simple decision to make him the highest paid QB in the league.