These are exciting, transitional times; I have no worries.
If we lost Walker and KDB, I would not be concerned.
With the former, he is a tremendous athlete but probably our weakest player on the ball. I think that Rico is capable of taking over from him. If that happens, we would have added ball-quality, and intelligence, but would, and probably should, look to replace his athleticism - Akanji becomes a more important player in this situation. Gvardiol's second season could see him emerge as the ball-playing centre back, a role he is supremely well qualified for.
KDB is a legend and still an extraordinary player. He will have to be used less, and will not be able to play 50+ games at high intensity. I think that Phil is able to replace him. I would shift Grealish inside to the left central position, alongside Rodri, with Phil further forward. It's second season for Doku and I expect him to be better again - a scary proposition. I hope that Oscar is given more game time and I believe that he can become absolutely world-class.
So, in the event that KDB and Walker leave, I would simply give Oscar and Rico more game time, allied to a shift of Jack into a more central role. Who we buy doesn't overly concern me - I think that we are well set up regardless. I haven't really seen a stand-out player in the Euros, but it has been a long, long season for so many.