99%???
C'mon!
There are parts of trails that you are up on the pedals in the lowest normal ratio on a geared bike, so unless you are gearing your SS at that it is just NOT the case.
Or is that 99% based on your normal choice of riding?
I have been experimenting with SS lately and would say 70% of my normal riding is practicable, I agree with improved technique and fitness that would go up a bit, but I don't think it would go past 80%.
Of course, I have absolutely no objection to getting off and pushing where needed. A lot of people going up the trails are doing just that with their downhill bikes.
Problem for a lot of people is that a low ratio is not practical for off hill riding, so a compromise is made.
Of course, you can just pick it up and carry it, it is a bit lighter after all.
But no amount of strength is going to allow you to finesse up anything steep + technical unless you have a decently low ratio, and then you are stuck with that if you have to ride home on the level.
So...
Flip flops.
But I consider a flip flop a two speed bicycle.
I would, wouldn't I?