That's true, and plenty of other teams have spent big fees on players who end up shit or underperform. Going back a bit, who'd have thought Torres would become so poor after his £50m move to Chelsea, or all the shite the rags have bought that turned out bang average. Benteke at Liverpool is another, £30m-ish and he's always on the bench.
I think some players just suit certain clubs, and a big transfer is no guarantee they'll repeat their performances with different team mates, coaches, expectations, etc.
We're better off going for the very best, who are so good even in off days they make a difference (players like Pogba, Bale, Neymar) plus get younger, hungrier players willing to learn from Pep and fight to make the team. Any who don't make it are cheap enough to loan out or get rid. The mistakes come when we buy decent players for superstar fees, which we seem to do a lot, then expectations are raised and they soon disappoint.