I don't think it would hurt sales, exactly, but I doubt it would be cost-effective to keep redoing them. People would still buy, but like you said, some people might wait, and that means the old ones go unsold, meaning Valve can't recoup that investment.
I mean, Valve has more money than they know what to do with, but I imagine they'd like to keep it that way.
It is also taking on technical debt, as each revision can come with OS quirks, and you now have to support X numbet of versions of hardware/software troubleshooting.
New deck wouldn't even be better. Even the handhelds that came after the deck (ROG Ally etc) struggle to offer something better than the deck.
The only things I could think of were a bigger display (that actually goes from top to bottom), max 1600x1000
which wouldn't even be an upgrade because the APU struggles even with 1280*800 with AAA titles anyway
and
Hm.
idk