Aguero has never been a 40 goal a season striker and I don't think anyone sane would have said he wasn't ever good enough.
Sterling without the midseason dip could reasonably hit 30+. Jesus as a starting striker could hit 30+. De Bruyne and Mahrez are both good enough to hit 15-20 over a full season. Foden with game time could hit 15 over a season. Bernardo knows where the net is too. Goals really aren't an issue. If we're failing to score it's almost certainly a team issue rather than a personnel issue.
Separately though, with regards to a 40 goal a season in England there's a couple issues. Firstly there's probably only 5 or so players that could do it at any one time. Secondly Pep rotates a lot at City, so no one striker is going to build up momentum like that without being astonishingly good. Thirdly it's really difficult to know if goals in other countries translates 1-1 in England. In France they play weak sides. In Germany so many games are goalfests, and in Spain if you're an overwhelmingly better team (eg like us in 17-19 or Liverpool 18-19) then because of the fact there's no goal difference tiebreaker there's a shit load of thrashings.
The rotation point is the stickler though. Aguero has a goal/minute ratio that is usually up there with the best. Sterling and Jesus are often not too far behind.
Given the rotation and game totals our attacking problems are more depth/variety than quality. It's further back (particularly at CB) the issues need sorting more urgently with player quality.