Agree with you on resources being better spent elsewhere. Sell Costa to fund our midfield and defensive rejuvenation. We’d still have Ronaldo, Dybala, Mandzukic, Bernadeschi, Kean, Cuadrado for forward positions.
He was quite good in Rome. And I don't think that's lack of mentality but more like money chasing attitude. Being in PSG doesn't mean he has no mentality, afterall they are in the game for the CL, despite their failure. Having Mbappe, Neymar and others confirms they are serious about it.
I think it's a very good signing for them. Price is, however, pretty high. But in nowadays market everyone has a high price even without a reason.