Marginal gains in sport is entirely a British thing, and they were well ahead of everyone else a decade or so ago. I'm not sure how much has changed but would expect them to still be at the forefront, even if they're not out in front by as much as they were.
Because if we accept that Salah will go when his contract expired, the season after next we will need to make up his 30+ goal contributions, plus he’s our only world class player in the forward line. Whereas in midfield we have two players who are world class in Thiago and Fabinho.
On your other point, If you believe the reports we are actively trying to agree on a new contract for Naby.
Two of the three players you've just mentioned are perennially injured and 31/32.
Next year they will be 32/33, Milner will be gone, Ox will be gone, Keita will probably be gone too. How can buying a midfielder not be a priority?