If you ask me, the Steam Deck’s battery is its biggest weakness, however it completely depends on what you’re playing. I can play Cyberpunk 2077 on the Steam Deck graphics preset for roughly 90 minutes but I played Arkham Knight for 30 minutes and lost only 10% of the battery. Dead Cells takes only 5% in 30 minutes. That is to say battery life is completely dependent on the game you choose to play, but at least you have the choice, whereas many crossplatform AAA games just aren’t on the Switch.
The portability difference is also notable. The Deck is a chonky boi, though imo it’s not as big a difference as some people would say. They both fit well in a backpack.
However, I completely disagree with you saying that the Deck isn’t a new platform. Nothing has existed like the Deck before because yes, it’s a gaming laptop that’s dramatically more portable and more gaming-oriented than any gaming laptop ever before. The control options alone make it the best controller ever invented (here’s hoping for a Steam Controller 2.0) and the flexibility to plug anything in via that USB C connector is fabulous.
Sharing my Steam library between my PC and my Steam Deck is just the icing on the gravy.