I just had a look. The listings I could see for the steamdeck had it at $800AUD minimum. For the switch I can buy it from a few different stores in my area for $370 AUD. That's very expensive and inaccessible.
I'm not trying to hate on the steamdeck. I think it's super cool and would love to have one. But it's too expensive for me and I'm not sure they even ship it to my area. While the switch is super affordable and a just more out of the box works experience. I know every game I buy for the switch will work great on it, meanwhile I can expect most games I have on steam will not have great controller support or may not run well on the system so its a bit more of a "at your own risk" platform.
Your issue is that you live in australia, where valve doesn't sell steam decks (yet?). So every steam deck listing you see is someone who imported it and is selling it at a premium since you can't get it in aussie land. I suppose not being able to buy one is a good reason to buy an Asus Ally instead, though, assuming you can stomach the higher cost and windows machine status.
I paid $350 canadian for a 64gb steam deck and paid another $100 canadian for a 1tb 2230 format ssd (thats only $80 now). I think the canadian dollar is slightly more valuable than aussie bucks right now but not double as valuable. 1 canadian dollar is $1.12 kangaroo bucks.
So your evaluation of the cost of the steam deck is just wrong. Its way cheaper than you imagine.
Also, valve has put literal years into Steam Input, which lets developers set up controller support, and also lets you load community sourced controller setups for your games. The Steam decks touchpads also let mouse driven games work pretty amazingly in a way that an xbox controller or switch can't.
You're just kind of wrong about the input situation. steam has basically the most sophisticated input software you can get and most of the time it just works, and when it doesn't you just load a community setup and play.
I'm not trying to hate on the steamdeck. I think it's super cool and would love to have one. But it's too expensive for me and I'm not sure they even ship it to my area. While the switch is super affordable and a just more out of the box works experience. I know every game I buy for the switch will work great on it, meanwhile I can expect most games I have on steam will not have great controller support or may not run well on the system so its a bit more of a "at your own risk" platform.