And apparently as early as 1981 airlines were doing that - this article is a gem. You could print it today, reduce the inches by one or two, change the model names and it's entirely accurate.

https://www.nytimes.com/1981/05/24/t...-reshaped.html

"When the Lockheed L-1011 first went into service in the early 1970's, most airlines started using it with eight economy-class passengers in each row. But as time passed, James W. Ragsdale, chief spokesman for Lockheed California, the Lockheed division that builds the L-1011, says, ''airlines decided they needed higher-density seating, so what you see today typically on L-1011's - or DC 10's, which are similar - is nine abreast in the economy cabin. And some airlines use 10 abreast; there is room for that, though the seats are a little narrower.''"

And the slim lines were a thing back then
"
A McDonnell Douglas executive notes that ''with the newer, less bulky seats, you might get as much legroom with 32-inch pitch as you would with 34-inch pitch.''

"

I don't know where it is but came across an article post Delta's purchase of the PanAm routes that it reconfigured one type of widebody because PanAm had an overly dense layout.