The Newton was first released in 1993. Not exactly "Many years" after 1989.
https://en.wikipedia.org/wiki/Apple_Newton
Plus, the GriDPad was $3000 and weighed 4.5 pounds (!!!). The Newton was $700 and weighed less than a POUND. Pretty substantial price (and SIZE! and WEIGHT!) reduction (over 3:1 for price, and over 4:1 for weight).
https://everymac.com/systems/apple/messagepad/stats/newton_mp_omp.html
The GriDPad was MUCH more like a tablet TERMINAL than a standalone computing device. The Newton was exactly that.
I wonder what the failure-rate in the field was for that CORDED Stylus?
Oh, and besides, the GriDPad was just a rip-off of the Linus Write-Pad from 1987:
http://oldcomputers.net/linus.html
Of course, it weighed a "LUGGABLE" 9 pounds!!! Imagine carrying THAT thing around...
Those are the precursors to the Ipad and the surface.
PDAs were the precursor to our modern day smartphones.