That's exactly what Chelsea and United have done...Chelsea, Kepa didn't work ok we get Mendy, ok Costa, Bats, Morata, Werner and now Lukaku in there striker department in the past 6 years. Now United, Bailey and Lindelof don't work ok we get Maguire and Varane who are also now not working, midfield oh we get Pogba, Matic and Mikhi ok not working lets get Fred and Bruno, ok maybe we can bring in Donny too, ok we need better midfield. Attack? Lukaku, Sanchez, Ibra, Cavani, Ronaldo, Sancho in the last 5-6 years, this is all just in the time frame Pep has been at City for those clubs. Implying City/Pep only can do this is laughable.
Hence why I said United and Chelsea problem isn't financially, its how poor the clubs are run. City have been chasing Pep since 2010, they've set up this project for him and now are reaping the reward. United can spend 100's of million year out and year in, but as long as they keep bring in bang average managers they ain't getting anywhere, likewise Chelsea there problem more is lack of patient with manager.
The situation aren't comparable in the way the clubs are run, not financially.