It's a good thing that Coutinho's now signed a new contract. Sturridge (2019), Lallana (2019), Coutinho (2020), Markovic (2019) are now all on long contracts. Markovic obviously has yet to fully convince, but there are promising signs, and then there's Ibe looking likely to play a bigger role soon - he's probably at the same stage of development as Sterling was two seasons ago (we need to look at his contract soon though, I think he signed his pro contract at the end of 2012).
So, even if we're forced to sell Sterling with 24 or 18 months left on his contract, we'll still have some promising or proven players in the same positions to make up for it. We're going to miss the threat he offers, but at least we're prepared for it and do not need to panic-buy to replace him.
The reality, right now, is that the richest / best-funded clubs will keep coming back to teams like ours to prey on our best players, and we (or rather, the players) won't be able to resist until we get ourselves on an equal financial footing. We can try to appeal to the sentimental pull of the club on the players, but the pragmatic way is to be well-prepared ahead of time.