That is 100% not correct. I’ve held both roles at a US carrier, it’s not nearly as simple as you say to keep people dual qualified. Which I why I mentioned this seeming like a good idea at surface level, but it falls apart when you look at everything the pilots and company have to deal with to do it.

And most unions have been quite opposed to tactics like this in the past, as management has used to it abuse right-seat qualified captains even more (by making them fly as FOs on trips that need it) in the past.

As to the first part, yes, they aren’t taking the position. It’s due to the way seniority works in the industry. While they may have a good schedule as a first officer, captains are generally more senior, so should they take the upgrade, they end up holding a much worse schedule than before. Normally the way to make up for that is a much higher pay rate. But it comes down to what each individual wants in their career.

As to the second, they could definitely keep people dual-seat qualified, but it would introduce an insane amount of complexities as well. Each seat would need a full qualification course, they have requirements for annual training that would increase, and anyone being told to fly as a first officer would not do it for less than their captain pay rates (generally due to contractual reasons). There ends up being way more to it than most people realize.

It’s a regulated position, as in they would have to be fully qualified as a captain before being able to operate as such in any capacity whatsoever. This sort of proposed solution would not really work.