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.
Feels a bit like the iPhone, there were some smartphones back then, but the iPhone took it to the next level.