Best case scenario is that this ends up a niche product. Realistic case is that it is super-niche. Either way, i say a maximum 2 iterations from Valve/partners before it fades away.
When looking at it as an entire package(including price) there is nothing appealing about this device at all. In theory, sure, its great to have a PC like device thats more portable. But this device is not bringing anything new to the table. No new user experience features, no exclusive games, no unique selling point. On top of that, it is entering a highly competitive market on the gaming side (Switch) and the portable computing side (laptops, iPads + mobile phones).
Its expensive considering its just going next to devices that you already own that do the same things.
I know the knee jerk reaction is to compare its specs to the Switch OLED specs and say how much more value but thats not how it works.
Nintendo prices their product on the value that they as a company bring to the market.
Valve is great at a lot of things but hardware isn't one of them. This is a device designed by PC building nerds with too much time/money without consideration of real market factors. Its not particularly portable and has bad battery life.
Also, imagine launching a handheld device after Nintendo and having an inferior screen.