I don't doubt grip-shifters function alright for what they are, and that some people would think they're great - but they're not for me. In use - I strongly dislike that one of the primary points of contact/control is also something that requires twisting at times I generally don't want to be moving my wrist around or changing grip on the bar. Looking at that shifter, it looks like it'd take either a very significant twist around to change more than one or two speeds, or would require several smaller twists to jump multiple gears at once (ie: riding tracks with sudden pinch climbs) - which to my mind largely negates one of the benefits of gearbox bikes which (as I understand it) is the option/ability to jump multiple gears during shifting without having to ramp up/down through all of them. Happy that it works for you (and others), but personally I wouldn't buy a bike (or drivetrain system) with a grip-shifter as it just feels like an inconvenience and a distraction while riding to me. From an engineering standpoint it seems surprising they can't manufacture a trigger-shifter option for it, it should be fairly simple mechanically and would really help take some of the stigma away from the change to gearbox driveline for many. Again, stoked you're happy with it, but I reckon it would be a significant deterrent for many...
You don't grip the handlebar there unless you're deathgripping. Your primary control point is from pinky to middle finger, preferably non-white knuckled. The entire handle moves on a moto-throttle and it fails to be an issue in application there. In this application we're talking a small section of the handle that isn't even a primary control point.
Trigger shifters are typically indexed on the shifter, and have a large opposite tension spring, so you only ever really load it in one direction and then release it in the other. The gearbox has the indexing in it in this case, not the shifter, and it requires two directional tension to gear change.
To create a trigger that has adequate force in the spring pull to engage the gear as it's released, you're going to end up with a bulky shifter that needs to be setup so the discrete ratchet points match that of gear box index itself and has a large, strong spring that is capable of engaging the gears in the direction it pulls (and only release on a successful gear change so you don't have a problem with the shifter index losing sync with the gearbox index).
The best solution at the moment for your trigger shifters on the Pinion comes form Cinq, which put a ratchet mechanism on the shifter cover on the box, and then has two separate paddles that shift up or down depending on which one you throw.
Shift to easy gear on left paddle, shift to harder gear on right.
As for the shifting, it takes about 330° to change from heaviest to largest gear, you can safely dump half your gear range on a quick single pull (165°), which I might add is 600% on the 12speed and 643% on 18spd versions of the box, without having to pedal the gear through.
Engineering wise, Occam wins out on this one. If anything a hydraulic shifter will be the next best bet as it doesn't require discrete intervals to be housed within the shifter. Any cable based trigger shifter is going to require some magic level of ratcheting in both directions or some kind of pulley with tiny indexes that converts a 70-95° lever throw into what is 330° of expected movement...whilst keeping a small form factor and light shifting.