This article includes sales estimates for different handhelds from market research firm IDC.
They place total handheld PC sales of the Steam Deck, RoG Ally, Lenovo Legion Go, and MSI Claw at almost 6 million units for the past 3 years. It’s estimated that the Steam Deck makes up between 3.7 to 4 million of those sales, more than all the other major handheld PC manufacturers combined.
The Steam Deck can behave like a Switch; if you want to stick to just Steam games with a big ✅ verified icon then it will probably be a similar experience, just with a different library of games (and better sales).
The Steam Deck can also be a full PC; you can plug in keyboard, mouse, screens, whatever, and use it to replace your laptop/desktop if you really want.
The middle ground is that the Steam Deck can be an incredibly versatile gaming machine. I can play the verified Steam games, I can usually play the non-verified games, I can install Heroic/Lutris and install all my free Epic/GOG/Amazon games with minimal tinkering, I can install emulators and run all my old Nintendo games at better resolutions and frame rates, and if I really feel like tinkering then I can install whatever else I want and try to get it working, but I always have all the reliable games to fall back on if I can’t!