Judging from your response you either: a) believe we need an entire new squad to make the finals, b) believe that we won't be trying to retain any players at all during the 2020-2022 seasons, c) don't believe that we will sign any other new players who come up for contract renewal this side of 2023, d) spend no more than a few milliseconds contemplating the full implications of what others say, or e) all of the above.
Retaining players is not a binary process. There are plenty of things that are negotiated during contract extensions. Having Thompson on the books helps nudge the spectrum of negotiations a little (or a lot in some cases) towards the clubs favour. Some players are going to get closer to the ceiling in the next season, gaining interest from rival clubs in the process, keeping them on the books just got a bit easier with the Thompson signing. Likewise future signings who prove valuable will be easier to retain.