Yes, I’ve seen the option recently (I forget which airline, but my instincts want to say “Air Canada” due to their reputation) where every economy seat is now a premium seat except the middle seats. It could be United, I believe they are pioneering the concept also of “pay extra for overhead bins”.
So it is possible, if you don’t book far ahead, to not have a seat to select. In which case, if someone comes along willing to pay more than you, you won’t get on. I have very rarely been given a different seat at check-in than what I selected when buying my ticket weeks earlier - but almost always I get what we selected; and at least the airlines have been smart enough to keep me and my wife together… mostly.
And another point - when I’m trying to book a ticket - Air Canada (mostly when I’m forced to choose them) offers flights in several categories; when the seats in “super-cheap” are sold out, then the next choice is “not so cheap” followed by “pay us a bit more” then “hold your nose and pay”. I try to get super cheap, and as I say, they didn’t bump us; even when they are auctioning at the gate for someone to give up their seats. So I’m going to conclude that you cannot keep buying the more expensive tickets and to keep bumping out already paid “super cheap seat” passengers. They have a plan as to how many tickets they will sell, and how many more tickets than the available seats, and when they hit those points they stop. Otherwise on days like US Thanksgiving, or Christmas, there’d be a large crowd of very disappointed cheap travelers at every gate as last minute expensive travelers bit the bullet and pay.
So I’ll agree - they don’t sell 21B (middle seat) twice. They stop assigning seats when they can’t find one that matches your criteria.
Oh, I’ve had this happen too. What happens is you can’t get a seat assigned when you buy the ticket unless you want to pay extra, and then someone (based on airline status) gets a free upgrade to the “Preferred” seat at the gate, and you get their seat.
Or, as happened to me last week on 2 flights is that the seat I was assigned at the gates were the “Delta Comfort” seats. They didn’t seem any different than regular seats to me, fwiw. On my return flight, there were at least 10 people waiting for seat assignments at the gate, which seemed like a fairly large number to me.
The tickets for last weeks trip had been bought ~1 month in advance, but weren’t purchased by me. I’d given the purchaser all information (full name, pre-check number, etc), but hadn’t thought to specify a seat preference, so I don’t even know if they could have selected a seat. But both flights were about 90 minutes, no connections, so it wasn’t a big deal to be in the middle seat on one of them.
Unlike the difference between steerage and first class, the “comfort seats” are IIRC something like an inch or two extra legroom. So sell the cheap seats plus the overbook limit, but don’t assign comfort seats for exactly as mentioned - people who buy “comfort” have a wider range of seats to choose from, get the ones they want; and then the airline gets to pick who gets the comfort seats at the gate, when they can see who are their more preferred customers (or someone chooses to pay the $20 to upgrade). But they probably have a limit, how many economy tickets - regular and comfort combined) that they will sell, since each overbook that gets denied boarding has to be compensated. And of course, you can’t buy a comfort seat if they are all spoken for…