The comparisons with Karius and Maignolet are weird, because they were starters. It's really hard to find a top keeper that is willing to sit years on the bench. We could pay well, but from the player's perspective it still means 1) not playing, 2) having a hard time getting a good contract afterwards. Becker is young and a top keeper, so there's really no realistic path for any keeper to get to the first team (except injuries). So we could get someone close to retirement or a young talent, but none of those would mean having a guaranteed top level option. Good players want to play.
Even if we had a better keeper, it's not easy to stay fit on the bench. For a Liverpool keeper, it's not even easy to get in shape by playing, since we often dominate so much.