We never properly invested after we won the champions league and never properly invested again after we won the Premier league the year after. You always need competition for places otherwise the players become less driven and the place becomes stale. Our starting 11 has been the same for 3 years now excluding injuries of course. Teams study before games, and after 3 years they know exactly what each individual from our team is going to do, they'll know the patterns we use to get our fullbacks in dangerous areas, they'll know the runs both Salah and Mane make from wide areas and that both like to cut onto their strong foot.. They'll know our highline will be not as effective without Virgil and Gomez as them 2 are the quickest CB pairing in the Prem and without them a quick striker will be able to exploit that area behind.
Like someone said previously we need to change formation or change the way we play, because after 3 years we've become predictable.