You are exactly right.
The club still operate the same way it did 15 years ago, the problem is the increased tactical sophistication means player profiles and how they knit together within the framework of a squad/system means that these opportunistic signings are nowhere near the advantage they used to be.
Holistically the club should be operating on a squad cycle timeline, be it A) Rebuilding, B) Talent, Quality upgrades, C) Covering gaps in an established squad.
Classic examples of transfer category A) Aaron Lennon, Kyle Walker, Danny Rose,
B) would be Alderweireld, Son, Wanyama
C) Pienaar , Llorente, Lucas
Obviously there will be variations within this and B signings can coincide with A & C. you adjust as you go along.
For every profile you should have a 3-4 player shortlist. Don’t deviate from the profile otherwise it ruins the system.
There should also be a contingency plan for every player already with us. The plan to replace them should always be there in case they want to leave.
Sadly this seems completely beyond the club management right now