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I think where I struggle to compare them is that the game gear was meant to be a mass market device that would sell tens of millions of units if not upwards of a hundred million. Handheld PCs have a narrower audience and tend to skew more tech savvy, especially in the case of the Steam Deck. They have much lower expectations for units sold because they’re not trying to do the “sell a bazillion units -> sell a hundred bazillion games for our system” game. In valve’s case they’re clearly creating a steam-dominated ecosystem that expands well beyond their handhelds.
I’m not implying they are uninterested in hardware sales. It’s that the model is different.
Current Nintendo does not care about an ecosystem. Each console/handheld they release has its slate of games they want to sell an average of per unit. They want to get as many units in as many households as possible and sell 6-8 years of games for that hardware with little library continuity between hardware releases. The switch 2 is the first departure from this tbh unless you count the Wii-u/wii which is valid but nuanced. This style best emulates all console/handheld releases pre-Gen 8 consoles.
Current iteration at Microsoft and Sony and valve is buy-in into an ecosystem that extends beyond singular hardware. Backwards compatibility, for instance. They want you gaming on multiple devices buying all games through their stores with your designated account that spans systems.
It’s a very different business model then what game gear et al were attempting.