You're right, it's not the general rule of progress of any team for that matter. United's latest improvement came not from expensive Martial, but from emergence of Greenwood and steady progress of Rashford despite his injury complications. Cavani helped a lot even though he's not long road solution and cost us nothing.
If Sancho goes to Chelsea, they'll have Havertz-Pulisic-Sancho lineup and they can give City strong run for their money. That kind of talent can make any team more potent to create and score. The thing is Football Association will probably give us another cramped fixtures next season and knowing our luck with injuries, we could really use rotation between Greenwood, Rashford, Martial and Sancho rather than fielding James, Mata or Donny on the wing.
I still have serious doubts if we even possess that kind of budget to go out there and just sign one player in one go and still have funds for other position, but this time we could finally have complete set of attacking players for both title race and Champions League to go past the group stages and try make it to the semis.