That brings up another question. What if Amtrak reduced seat pitch and put in some more seats? I would be okay with less pitch if Amtrak was willing and able to speed up their timetables.
I wouldn't. There is such a thing as "perceived value", which is marketing 101 and it's also something most of the airlines are terrible about, and probably the main reason they're doing so poorly right now.
When you downgrade service quality, you also downgrade the amount that the public thinks that service is worth. And this is a losing battle, because as everyone knows, the higher the prices you can charge, the greater the profit even if your margins stay the same. The best business strategy is almost always to find a way to charge the maximum amount possible while still running at maximum capacity. Increasing capacity while decreasing revenue per passenger is a terrible strategy, as the airlines are finding out.
It also turns into a vicious cycle, because the lower your revenues, the more services you have to cut, which means the lower your fares go, which means more service cuts, ad infinitum. Amtrak would be somewhat immune to this because they're subsidized, but they'd still probably have some 'splaining to do to congress about why they're carrying more passengers and making less money.
Amtrak's seats
are obscenely big, so I wouldn't have a problem with them using some thinner seats at the same pitch and squeezing in another couple of rows that way. I'm hoping that's what they're doing on their new cars (I've seen the seat pitch of the new cars, but I'm not sure what the seat pitch of the Amfleet cars is to compare it). But anything that degrades quality is going to decrease the fares they can charge, which is never a good thing. They should be finding ways they can
increase fares without losing customers, and that means improving service. Look at Acela Express, which is high speed and all BC and FC - it's very successful.
I'll just say also that in Japan, where people are shorter on average (and pretty much everything smaller than here), standard coach seat pitch on most shinkansen train sets is equivalent to first class seat pitch on Acela Express. The seats themselves are a lot smaller, but the amount of legroom is tremendous. And people pay a premium to ride those trains vs. flying.
Edit: oh, I almost forgot! About those seats in the original post - they *can't* ever happen because most airliners are already configured to carry the maximum number of seats for which they're certified, and there are all sorts of impossibilities about recertifying airplanes for more seats (evacuation rules, maximum takeoff and landing weights, seat strength, fuel efficiency with the extra weight, etc.). So this will never, ever happen, on any airline flying modern, large airliners. Maybe it could happen on some light, overpowered prop planes that otherwise only carry 15 people or so, and they want to squeeze in 3 or 4 more.