700 c is more easily interchangeable with a stock tubular rim.

Otherwise, I think you're right...27's work fine if you put alloy
rims and good tires on them. Maybe even preferable in some
applications.

As people have accepted the idea that more speeds = better,
the rear cluster has grown from five to ten and counting.

I think I saw the first six speed rear clusters about late 70's,
but I'm told some people had them earlier. All those cogs gotta
go somewhere, thus wider spacings.