I wouldn't call the steam deck the greatest piece of gaming hardware ever. There are many very well equipped PC handhelds out now. The difference that Valve brings is that their software is impeccable and they have a good history of supporting their hardware with up to date software for many, many years.
While there is better hardware out there now I can guarantee you that the software sucks on all of them in comparison to the steam deck. And if you run HoloISO on them you won't get the full feature suite.
Plus of course the price point. You can get a refurbished SteamDeck for like 330€ now. While alternatives start at like at least twice that for almost the same performance.