Where are you getting tacos where they are too gigantic to fit inside a normal human mouth? I wouldn't buy something like that either. Around here even if you get a "super taco" it is only longer than a standard one and can still easily be bitten into even at its tallest while getting toppings and meat. Just don't shop at "entire burger sized tacos r us" and its a much easier eating experience.
I'm seeing a lot of complaints about messiness in general. Too much I think. Maybe I'm just being too ableist, but human beings have very complex hands with fingers capable of fairly deft movements. Someone who can't hold and eat a taco without winding up covering themselves and others around them with its contents... maybe should see a doctor about that. As for messy... that's why you eat it over a plate. Like a person. Anything falling from it then lands on the plate. The fallen morsel can then be picked up and then eaten provided it was a clean plate. As for walking around, street taco style. Do that outdoors, the place that you can see sky overhead and ground underfoot. A crumb beef or shell or stray shred of cheese or lettuce falls... it lands on the ground... don't worry. Its biodegradable, and generally will be eaten later by some kind of critter before that would be an issue anyway. Maybe its just a "land of the blind" kind of issue, but I don't seem to have trouble navigating the the treacherous balancing act that is eating a taco. Am I just underestimating my incredible manual dexterity... or is it just people overestimating the difficulty of what I'm judging to be a fairly routine and easily accomplished task?
I'm being largely facetious in my "hatred" of tacos, but yes, hard-shell tacos are indeed inconvenient to eat. By your own admission, if a food meant to be eaten with the hands requires a plate to catch the inexorable fallout, then you've got a design flaw mostly solved by soft-shell options or burritos.
And I'm not sure where YOU'RE buying your tacos, but I know in my case, the average hard-shell taco, at its apex, tends to be wider than a human mouth, so an appreciable bite of cheese, lettuce, tomato, meat and shell is very difficult without appearing a glutton trying to stuff his/her face instead of eating like a civilized person. And name another food that, by design, requires the consumer to turn their head sideways.
Point being: hard-shell tacos are simply more work and more mess than they're worth when you can wrap the same ingredients in a soft corn/flour tortilla, or the easiest solution wrap the ingredients entirely as a burrito, translating to "little donkey," as in the domesticated working animal used to lug bulky stuff for ease of use and access for its owner.