SteamOS is the important part here - if it is proven to be a good console experience (which the deck has basically proven already) then licensing of the OS to other manufacturers will put a lot of pressure on integrated h/w s/w manufacturers.
Unlike the handheld format, the tvbox console is fairly easy to manufacture and is tolerant of a lot of spec and price variety. Any slip up by Sony and Microsoft in specs and price will result in steam machine variants carving away market share, which could force more frequent console releases.
The steam machine will almost certainly come in at a higher price point than the PS5, but with no 'online' subscription charge and reasonably priced storage upgrades we may see these revenue streams disappear from the next console generation in order to compete.
SteamOS isn't perfect, and the variety inherent in the platform that is a strength is also a weakness. The core markets for Nintendo and for Sony aren't going anywhere.