Piltdown Man
I like your logic but it does have a slight floor, in that the measurements must be as the a/c is equipped for service.
This means the seat has to be dressed and the literature pock has to include what an operator places in it for the pax.
I witnessed many years back a B767 that was reconfigured to its max seating figure.
The a/c was checked after the change by the local CAA surveyor and he would not approve the change, because we were not meeting the seat pitch measurements.
He eventually allowed the a/c a dispensation to operate for a limited time, this with the conditions that the in flight magazines and headsets were not allowed to be in the pockets for take-off & landing.
I read the flight reports from the cabin crew and they rightly pointed out if they did have an emergency evacuation there would have been carnage. This due to the the magazines were strung out all over the floor.
This type of problem is why seats were developed into the shapes that they appear today.
If you look carefully at the newer types of seats on "IT" or low cost operators you will notice that the back of the seat is shaped or sculptured with a curve or reduction in foam at the critical point where the measurements are taken.
Thomson was one of the first operators to start fitting the type of seat with the pocket behind the seat table and Jet2 have followed.
I have read reports that the seat is comfortable for short flights, but some say they would not like a long trip seated in them.
I also have to disagree on your assumption with numbers of seats and greedy operators.
The max number of seat you can fit to a B737-800 is as you point out is 189 and that is also the same for the B737-900.
Most "IT" or low cost operators operate at 189 and l can assure you they would fit more seats if the could.
Why do you think the B737-900 has not sold in Europe to the type of operators l have mentioned. It is because you can not fit any more seats even though the a/c is bigger.
I am sorry to say that Joe public wants a very cheap flight and that is why we get these terrible uncomfortable seats.
Yes l do understand that Bsmasher has a reasonable gripe, why are his favourite seats no longer fitted, l am sorry to say it is progress.
It is seat manufacturers meeting the demand from the airlines to reduce cost, weight and meet what the airlines want.
So l would guess you will find very few economy seats designed the way they were and if so they would be more expensive to buy.