Buston Posey has averaged over 5 fWAR for the past 10 years (or, 9 years, as he sat out 2020).
Of course, Posey has not actually had a 5 fWAR season since 2016. And of course, the Giants have still been paying him to be a 5 fWAR catcher that entire time.
Those long term contracts do have a built in back end of potentially extremely subpar performance. And everyone is willing to live with them until they get here...