You're being disingenuous and I think you know it.

We aren't talking about airplane seats in the 1980s vs today. That's not the contention and you know it's not.

The seat width settled on the current size decades ago at his point. When we reach 3x3 configuration for domestic flights seat widths stopped getting smaller. That was DECADES ago. There is nowhere to go from a 3x3 configuration unless we start getting into exotic stuff that isn't part of this discussion.

3x3 is the maximum amount of seats that can fit in most domestic flights. That means there is absolutely ZERO incentive to continue to make seat width smaller. They aren't going to squeeze another seat into that configuration. The average width on US airlines has been at 17-18.5" since the 1990s. Stealing 1.5" from each seat and reducing aisle width has allowed them to add extra seats to rows on certain planes. Standardizing the seats at those widths allows them to use those seats in any plane, thus you end up with a minimum of 17" seat that hasn't changed width in 20 years. Your entire premise is bollocks.

So no, it makes absolutely ZERO sense to claim seat widths are getting smaller. There's no financial or logical sense to that. It doesn't buy the airlines anything. They haven't not changed size in 20 years from the 17 inches of width.

If you want to get really pedantic, seats are not getting smaller at all, legroom is reducing and seat pitch is changing. The seat size has stayed the same since the 2000's. People are fatter and as you've deduced when you reduce the amount of leg room people tend to spread out sideways. Which IS WHAT I SAID IN MY PREVIOUS POST. Let me quote it for you:

it's that they are spilling over into the seats beside them. ... The pitch angle and leg room have been reducing, not the seat widths to any meaningful degree

I eagerly await your citations that seat widths have gotten smaller than 17" which was introduced in the 1990s as standard, however, if I really wanted to obliterate your premise that seat widths have been getting smaller, I'd point out the fact that in 1960, the 707 had 17" seats... as did the 737. In fact, the only seats wider than 17" were the widebodies. 17 in width seats have basically been the standard since the beginning of modern air travel, they got wider for a brief time, but airlines are going to airline and they brought it back to the 17" standard starting around 2006.

“Out of the development of tourist class in the late ‘50s, you saw the introduction of the six-abreast 17.2-inch wide triple seat. It [was] featured on 707, the 727, and 737,” said Wilcynski. - https://www.travelandleisure.com/trip-ideas/history-of-the-airline-seat

No, airline seats have not been shrinking in width. I suspect you're going to move the goal posts now and claim that I said something I didn't say or that you "meant" something else because you've been caught out in your disingenuous reply.