There are many things that set the Steam Deck apart from its competition: its unique SteamOS, its custom AMD APU and an unbeatable starting price. That being said, there are still devices around that will give it a good run for its money, depending on your use case and budget. Here are some of the contenders.
https://www.notebookcheck.net/3-pricey-handheld-PCs-the-Steam-Deck-has-to-fight-for-supremacy.606812.0.html
If those are pricey there is no fight. High prices means lower sales numbers. If Valve plans to gain money from the hardware they have chosen the wrong business model. The portable should bring users to Steam, not the other way around.