I genuinely think Keane will prove to be a great signing in time.
However, I do agree with the general point you made about so many signings with so little apparent forethought.
I don't agree that the players are mediocre but certainly good players can look average in a team that is playing badly and low on confidence.
I think that individually the players are quite good, but there seems to have been a scatter gun approach to signings while at the same time not replacing the players that the manager knew were leaving... Lukaku and Barkley.
I think that Sandro was certainly worth buying at the price, Pickford now seems cheap, Vlasic was at least value for money and I think Keane will prove the same.
Klaasen seems to be the one that we will really take a hit on and even though it could be said that Sigurdsson cost 20m too much at least he is in the first eleven every week and certainly is working hard enough for the team. With the way prices have gone crazy he will probably appear very good value in twelve months time.
I would look back at Chelsea in the last season that Mourinho was in charge and the players looked so ordinary. Twelve months later under Conte many of the same players looked like world beaters.
Players need confidence in the manager and in their system , they need confidence and belief in themselves and very often that is the difference between looking very good or very ordinary.