I think you meant to say "Deck" in the second paragraph.
But yeah I totally vibe with your observation. Something a bit ironic with this situation is that a big part of why other companies simply can't provide the kind of service Steam does is copyright issues - XBox and Playstation both give out free games, Nintendo has their online service, but no option remotely compares to "make everything available on one app on the most modern device." Imagine if Nintendo put everything that had ever appeared on the Wii/DS/Wii U/3DS/Switch shops all on one online storefront on the Switch, and let you attach ownership to your account and play everything you owned on the most recent device - then they would have about a quarter of the functionality that Steam has on the Deck, where you have access to every game you've bought for PC for as long as Steam has existed (and quite a few things from before that) and the number of things that have lost compatibility is pretty low.