Which implies the money is the only thing of interest to us...
How about the hole he'd leave behind?
We either replace him by buying someone (presumably costing less than the Walker fee), or Poch feels that we can get by with KT and KWP. Either way, in Poch I trust. Especially seeing how he's completely changed us from serial goal conceders to the meanest defence in the PL.
How about how much he'd improve a direct rival (to whom money is no barrier)?
Walker before Poch appeared was a pretty average PL RB. Very pacy, not much else. Without Poch, what sort of RB would they be getting?
How about the message the saga sends to the other players?
Yes I agree here. But there's also the point that if you put Walker on £100k a week to keep him, there are also several more valuable players who might get the hump as they should be on more than him (Toby, Jan, Rose, Kane, Dele, Eriksen, Hugo, Moose.....)
On balance I'd prefer to keep him - for the stability. For once our top XI would be the same as the previous season, the continuity would be great. Play Walker against the stronger teams, rotate with Trippier for the weaker ones. But I won't be devastated if he goes. imo he's our weakest first choice defender. If we have a weakest link in the team it's probably him (or Hugo's kicking).