Back in the Bad Old Days of the 1980s, I used to travel with a separate folding luggage trolley, as did many others. They did not make reliable wheeled luggage then.

I used to put the luggage trolley in the overhead bin, sideways, taking up the space where, today, two rollaboards would go. No worries - there was plenty of space with lighter passenger loads and free checked baggage.

Second personal history:

I had the displeasure of flying Southwest, once, and the overhead bins were jammed. I got to an empty seat and found that space in the overhead bin, which would be plenty of room for my rollaboard, was taken up by a PLASTIC RADIO. It was a small boombox. What kind of idiot would put a fragile plastic radio in an overhead bin with lots of heavy, fragile-item-smashing rollaboards?

I just pulled the radio out and put my own bag there, intending to put the radio in front of my bag. The owner said "Hey, don't touch my radio. It's fragile!" I replied "What is it doing in the overhead bin?" and handed it to him. It fit, easily, under the seat in front of him. The thing was no bigger than a Kleenex box, and he just HAD to put it in the overhead bin, with no padding or other protection, taking up the space for a rollaboard.