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.