I really don't see this as problem, and most of my gaming these days is done on portable platforms. Handheld devices come in a few form-factors.
Pocketable: DS/3DS, PSP, VITA, Switch Lite (borderline pocketable), the smaller form-factors of GPD Win, etc
Baggable: Switch, Steam Deck, larger more powerful portable PCs, etc
The Steam Deck isn't too big that it doesn't fit in the second category.
Most of the time I am on the go I have a bag/backpack. If I am visiting my partner's house, on my way to work via public transportation, etc. I have a bag with me. So I really don't care if the Steam deck is somewhat larger than a Switch or even Wii U Tablet. As long as it, within its case, fits in my backpack and isn't as heavy as a gaming laptop that works great.
Furthermore, the Steam Deck acts as a fully functional mini-PC. When I go to work, I don't often use my laptop, but rather a NUC (due to better cooling for CPU-intensive tasks) instead. I can easily see myself using the Steam Deck for that purpose and leaving my NUC home while getting some of the advantages of a laptop and being able to play games on my commute.
Sure, the Steam Deck is a niche device, but if you're in the niche (as I am) it is a pretty interesting device. And I really don't see how it will fail given that reservations sold out very fast.
