So to reiterate, it's not that Walker is some horrible unqualified coach. It's that failing to extract value from that resource has a cascading ripple of consequences on everything else.
It's possible that Underwood sees value that you don't in Walker outside Tilmon and Smith. It would be naive to argue that Walker's value wasn't in part tied to Tilmon and Smith but not inconceivable that Underwood valued Walker far beyond that. It's also possible that Underwood didn't value having the "system" guys from Ok St as much as you do.
It is not as if Underwood is some shrinking Violet that would allow Whitman and other influences to force him into a staff he's not comfortable with. I am confident that if Underwood thought Evans was essential, he would be here.