Nobody; so it would be your word against theirs and if you were dead they would probably win!
It is based upon the passing conventions to give the pilot a better view of opposite direction traffic. Conversely; why do we drive on the left in the UK? Well, its because Coach doors were always on the left. Why you might ask? Because most people are right handed and when drawing ones sword for protection, it was best that you keep potential enemies on your right and you unprotected side to the left. A pitty boats didn't have doors in the side because aeroplanes opted to follow the nautical convention. I gather some NASA vehicles were based on the width of a railway track which was determined by the spacing of charriot wheels that was based on the width of a horse's arse.

SRBs. Bollox, of course, as the US rail gauge is same as here; 4' 8.5". But their trains are much bigger. So the chariot wheel / horse's arse theory doesn't follow, as there's no direct relationship between rail gauge and loading gauge.

Back to aviation - why do aeroplane captains sit on the left, and whirlybird ones on the right?

I thought it was because the Yanks invented powered flight & they drive on the right sitting in the left seat they made the rules that way round?

Concerning helicopters....

Firstly, the pilot doesn't sit on the right in all helicopters; some are flown from the left, some from the middle. It is true that most are flown from the right. I've heard several explanations, but the most accepted one seems to be that the first helicopter pilots were taught by fixed-wing pilots, who sat on the left, so the student sat on the right, learned to fly from there, and continued to do so.

When flying solo it's sometimes a weight and balance thing; for example, the R22 can't be flown solo from the left.

Yes but the size of the tunnels a train has to pass through is determined by the max train car width which was influenced by the track gauge. So you could say that ultimately the size of Roman Horses rear end was a factor in the diameter of a rocket motor if they wanted to be able to transport it by rail.

I was taught that the horses backside story was bollocks anyway.

That the gauge of these things was just a trial and error initially.

The wider you make it the more stable it is but the harder it is to get things to change direction.

Over the years that width was found to be the most useful at achieving carts and wagons that don't fall over in a gust of wind but also you can get them to turn corners as well.

Hundreds of years later we found out the engineering principles and maths to prove what wagon makers had known for hundreds if not thousands of years before.

People have tried to reinvent the wheel when it comes to that gauge and it never works quite the way they intended.

There is a whole heap of geometric ratios out there that humans have used since before the pyramids were chucked up for designing things.

Cultures which there is no proof even knew about each other or even knew there was such a thing as a horses backside where using the same ratios and gauges for transport sledges and things like that.

MJ

Very little changes in aviation at more than a snails pace. Ok the electronics have changed but the basic airframes are still very much like they were 50 years ago.
We have had the Canard idea but aircraft still have conventional wings, fuselage, tail with fin and elevators, a nose wheel and two mains.
The seating could change to a central pilot seat! Why left or right? or even staggered seats ( again tried) but we still end up with aeroplanes that have not evolved from what we had 50 years ago.
Even the engines Continental or Lycoming are things of the past so its only computers and sat nav which have brought changes.

Someday someone will come up with a revolutionary lifting device or configuration but till then NOTHING change

pace

You can't reinvent the wheel Pace. An aircraft is the shape it is because that's the best shape for it. As for middle seating, McClaren tried it in their road cars and it wasn't really liked. Plus would you really want a middle seat in the typical GA four seater? The passengers either side would have to be like Kate Moss.