I think the Knicks are better run now and won't consider taking on Westbrook even if the cost were nothing. Westbrook would help make their future draft picks worth less, and eat up shots they want their young players to take.
If it were earlier in free agency and you knew an additional star would be a combo deal, I could see them being willing to do it. Westbrook solo isn't really worth it.
The idea of trading him for Wall is fun but the four years is really tough. If you are a rebuilding team, I could see taking on a 3-year deal just for assets, but 4 years? Also, I expect Washington to begrudgingly start a rebuild sooner rather than later. Having an unhappy Westbrook would be worse than an unhappy Wall, since Wall can't really play anymore.
Perhaps Chris Paul is an option? He's "only" signed for three years and the Rockets are desperate to stay relevant. You get one year less on the supermax deal and maybe a future bad first rounder.