Next season the 'first team' may well be 18 - 20 players (plus 4 genuine squad players) given the amount of rotation required - hence 5 subs per game. All due to season being one month shorter than last season .... but same number of games so most weeks its playing midweek so no rest = rotation throughout squad.
So its improving top 18+ players not top 11
This doesn't escape the simple fact that we can only field 11 players and that, if we want to stand a better chance of beating better teams, then we need to improve the weakest areas of said XI.
We could sign Lewandowski to rotate with Kane, but that doesn't make us any more likley to beat an elite team in a one-off game and those weaker links would still remain.