My guess is that this poem refers to
The number of wheels on automobiles
Title: This poem goes downhill quickly
Things with wheels sure would go downhill quickly
At first there were none.
It wasn't upsetting (though I sure was fretting), but not even one!
To start, no automobile had been invented
And then there were four.
I wasn't controlling while we were patrolling, but then there came more.
The first wagon was invented - but it was horse led. Then came cars with 4 wheels.
I really liked three!
Drunk with elation (my own destinations!) but I wasn't free.
3 wheeled cars were then invented by Karl Benz
Then, OhMyGod, two!!!
Screw "reprimanding," my future's expanding, boundaries eschewed.
The motorcycle (motorbike) was invented with just two wheels.
The best of all? Four.
This isn't regressing; the world I'm accessing, and my feet aren't sore.
Despite going back to the trusty old 4 wheels, the newest sports cars were invented and cars now certainly allow you to travel quickly without walking
Then, an uncomfortable four.
Most likely a bus - which can be very uncomfortable
A constant reminder of modes that were kinder.
Freedom abduction that leads to obstruction.
Buses certainly make me at least wish I was in a car...
A constant restraint that's not good for my taint.
Soon there'll be none once more.
We now have aircraft, which don't need wheels and the invention of flying cars may be very soon.
Could be way off, but hopefully this is quite close...