There were many years between the Steam Controller (2015) and the Deck(2022). I’m not sure how much of those learnings could still be applied, considering changing technology as well as staff changes.
Maybe Valve has such excellent staff retention that this would be a non-issue. At my previous employers, two years would have been enough to have to start over from 0.
First the steam box then the controller, I’m sure they learned a lot from both of those before they released the deck.