Because of the league's structure that aims towards parity, every expiring position of strength can't always be equally replenished, at least in the short term.
What exactly the Rams are going to do for 2022 at LT I don't know. I feel confident they have a pretty good handle on it. However to think that the Rams are going to be fortunate enough to field a LT as good as Whitworth might be a bit optimistic.
Just as the Saints can't field a QB in 2021 as good as Brees even knowing he was soon going to retire, it's that way with the retirement or FA loss of some exceptional players for every team.