In many ways this is clearly their second-take on Steam Machines. They learned the first time around that licensing the name to third party hardware vendors didn't work (a lesson they could have learned from the 3DO), and that people didn't really like streaming games from PC to TV.
To me this seems like a decent combination of all of their past hardware experiments. It’s a steam machine, but in a portable form factor. It has the streaming capabilities of the link, and the touch controls of the steam controller. If this takes off, I’d expect to see them continue to release accessories and iterations of it. Maybe we’ll eventually new steam controllers for use in the docked mode.
Steam Link and Steam Controller were dropped because they weren't selling very well, but the steam link application continues to be updated, and the Steam Controller continued to get firmware updates until 2018.
If this Steam Deck thing sells well, they'll probably make another one. If it sells poorly they'll probably keep the software updated and use put discounts on it to clear out stock.
It was pretty clear why they dropped them, which is most people hated it (while a few very much loved them). I loved my Steam Controller but right now I'm using mostly my Xbox controller for PC.
The Wii U was a failure, but I feel (and I guess the sales can confirm that) that Nintendo learned from their mistakes.
I would like for once for you to keep working on a singular product, release multiple iterations, make it better and make it the market leader.