That's true we will need a replacement sooner or later but that's 2 years off... there is not just one scenario that can happen, I know what people are fearing: Mbappé goes from strength to strength and starts looking every bit like a future Ballon D' Or contender in that time and nobody else emerges or prices continue to rise and we can't afford a player that does emerge.
As Pep alluded to in his latest conference it simply can't keep rising at the rate it's doing something will have to give... the adage that players are only worth what clubs are willing to pay works both ways(clubs will start talking themselves out of deals sooner or later asking too much... if Real Madrid are spitting their coffee out at what's being asked I think we need to take notice). If we miss out on Mbappé I think we will be fine personally, we can take it easier next summer on our spending and save up for a splurge(unless we unearth a gem or an academy player really steps up) when Aguero does leave.