Is there really any reason to extend a guy in the middle of a rebuild?
depends on what kind of rebuild you think the Nats will be doing. I don't think / expect them to do a 3 year hard tank to accumulate high draft choices like the Astros and the Cubs did. I think there are too many non-competitive teams to make it a lock you'll get 1-1-6 three years in a row like 2009-11 (IIRC).
I suspect they will be a top 10 payroll through 2024, at a minimum. The Nats don't have much in the way of commitments next year (Strasburg, Corbin, and Will Harris, and an option on Schwarber), so they will have a lot of flexibility to sign some guys like Scherzer who will keep you competitive while looking to hit on a few more Harrison / Gomes types to surround Soto and maybe some other internal pieces over that period. Sticking my neck out, I'd say they may be able to go cheap on the bullpen during a rebuild if Finnegan, Clay, Voth, Suero, and Rainey are their core.
IF they are a top 10 payroll, then there's probably some room to re-sign Scherzer, retain Turner next year at least at the start of the year, and, with a few signings that work out, have a shot at competing like the 2013 Red Sox or the 2019 Nats and maybe the 2014 Giants. Not the worst strategy.