Is Chelsea fall inevitable?

If you have noticed Chelsea for the past two to three years, they are selling big players likes of Mata, David Luiz and now Peter Cech and buying average players like Salah and Costa.

Does this mean, Roman's blank cheques era is over and they have to sell players year after year to compete for the top4 finish just like Liverpool? My question is, who will take their place? Is it West Ham or southampton?