I had thought when the Segway came out that a wheelchair version would follow shortly afterward where a person without functional legs could get on a saddle and move around quickly and at a typical human height so things on shelves aren't such a problem.
Segway hit the market 20 years ago and it still has not happened. I get that there are a number of complications (getting on and off is a challenge), but it seems like it should be solvable. The technology has only improved over the years, especially the batteries. This should be doable.
From my own (admittedly limited) experience with riding a Segway and my own (day to day) experience with a child that can't sit, stand or walk, I would guess that it is a) difficult to get in and out and b) difficult to balance and steer.
Extra mechanical stabilizers are in the way when you finally are on/in and without the Segway was really stubborn and threw me and others off in the beginning. A person without proper sense and ability to balance, will have big problems getting onto it and off again with just the gyros running. Also balancing is a really difficult thing to do if you have the problems that forced you in a wheelchair in the first place.
For people who are ok with the upper part of the body, and can lift themselves onto the machine, you might be right though. Unfortunately there doesn't seem to be a "used Segway" market that is cheap enough to attract hackers trying out new things with them.
Not long ago I saw a guy in a Segway-like wheel chair. It looked just like a regular wheelchair, with two wheels on each side. I spoke the owner and he loved it.
Here is a Segway converted to a wheelchair here in New Zealand:
Segway hit the market 20 years ago and it still has not happened. I get that there are a number of complications (getting on and off is a challenge), but it seems like it should be solvable. The technology has only improved over the years, especially the batteries. This should be doable.