I am a huge proponent of the steam deck but there are a few things you’re kind of leaving out here
You would be surprised how many people actually don’t have a mouse on hand. But if they do, you do have to calculate that into the cost.
you need a dock or at least a cheap USB-C dongle unless the only thing you plan on hooking up to the steam deck is your mouse, which would be feasible but unusual.
The deck is getting a little longer in the tooth. I love it, I use it a ton, but it is definitely starting to show its age.
It can’t play Nintendo switch/switch 2 games natively, which many people who shit on Nintendo (I am one of those people don’t get me wrong) tend to forget is a pretty huge part of the appeal of Nintendo platforms.
if you plan on doing Couch multiplayer, the steam deck is really not a viable option compared to the switch unless you invest in a bunch of other stuff
The steam deck can get fucky with 2 Bluetooth devices (forget 3) so unless you plan on a M or K you need a dock or dongle. I guess unless you plan on not charging either.