Kane is an upgrade on Aguero if Aguero isn't playing. Prior to last season I'd have taken Aguero over Kane, but only just - so it's probably taking us back to a similar place that we were in for most of the time Pep has been manager.Last season:-
Aguero, 6 goals, 1 assist
Kane, 33 goals, 17 assists
Silva, 5 goals, 9 assists
Grealish, 7 goals, 12 assists
Much as I love Aguero & Silva both, that looks like an upgrade.
Yes, Agureo didn't play much but that's another negative surely? Kane played the equivalent of around 36 games more.
Grealish played less than Silva BTW, around 11 games less but with better stats in a (much) poorer team.
Bernado wants to go by all accounts and time does not stand still, not even for Sergio Aguero.
Not sure about the other comparison though - Bernardo clearly doesn't play in the same role as Grealish. Jack is in a more attacking position, and is the "give it to Gordon" player on his team.