The centimeters we're speaking of has no effect at all at how quickly disease spreads, the way the air circulates is more important than your leg room.

Seating like we used to have a few decades back is available. It costs about the same as it used to. People seem to forget how expensive air travel were back in the days, no wonder it was more luxurious.

I haven't seen anywhere suggesting that the transmission vectors between passengers are a major cause of the growth into a pandemic - even if that was the case, I doubt a once in 50 year disease is cause for them to change to a less profitable seating setup the other 49.

There are lots of cases of diseases spreading from one plane passenger to another.

That's part of the reason the airline requires you to sit in allocated seats - so they can notify the people sitting in neighbouring seats if you have a notifiable disease.

My personal hypothesis is the low pressure ultra-low humidity air in planes dries out mucus membranes in the nose and throat making it more likely for airborne things to get in.