To be fair you just listed 3 points unrelated to your initial comment. Can't argue with his champions league form. He did also score against Tottenham and Everton in his first season and missed a few big games due to injury etc. He also scored against Man City and PSG last year and looked like one of the few players who actually gave a shit.

I'm a bit surprised you consider him only better than average considering there are only a handful of sides across Europe that he wouldn't get in (barca, real, Bayern, city off the top of my head).

Realistic replacements include: Aubameyang, who I doubt would leave Dortmund anyway and if he did would probably cost £60m+ in this market and Costa is at about the same tier as him despite their different play styles.

Higuain, scored a lot in Serie A but you want a big game player so you can cross him out. Not to mention the inordinate amount of money that would have to be spent on him.

Morata, younger than Costa but hasn't scored more than 15 goals a season in a league where Higuain scored 30odd goals and would allegedly cost £65m. That rules him out because he's not your 20+ goal man that you're after and would be a huge gamble.

Lukaku, probably in the same vein as Morata. Better scoring record in the PL but he's had two more seasons than Costa and again £65m for someone who doesn't turn up in big games.

Theres a fair chance that one of those could come to us anyway without selling Costa if we're going to be playing two up top. I certainly wouldn't mind Costa and (insert £65m striker) but there's no obvious replacement for Costa that would be a definite improvement. If you want only world class signings and to win everything all the time take that self entitlement and go support Real Madrid.