If you could flip a switch and the segway could go operate in the 10-70mph range, and flip it back to operate in the 0-10mph range, you could transition from street to sidewalk (vehicle to pedestrian.)
This would be worth the extra money and not be uncool.
Even at 70mph you'd still run into the problem pg expouses - looking like an effortless dork.
As for this bit - "imagine something that worked like the Segway, but that you rode with one foot in front of the other, like a skateboard. That wouldn't seem nearly as uncool." - have you seen the Scarpar? It's only 35mph, but definitely, definitely cool.
are you serious? this may be what the segway needs. Extreme segway sports. If i saw someone going 70 mph on a segway, my first thought would certainly not be 'what a square'.
... maybe. Looks a bit wobblier, but look at it in context - they are on a cycle path and they fit in perfectly well. Someone on a Segway would stand out quite a lot.
This would be worth the extra money and not be uncool.