And the answer to your first paragraph is to give them the good schedule of first officer for three weeks of the month and the bad schedule of captain for one week. Pilots may be willing to tolerate a crappy schedule for part of their month instead of the whole month.

And the qualifications of being a captain flow from the experience of being a first officer on an aircraft. In order to be a captain on a flight, you need to qualify to be a first officer. So, it isn't like United has to pay for maintaining two separate certifications, just the more onerous one. And you make the dual flight role optional. You let pilots decide if they want to fly in a captain role at captain pay all the time or if they are willing to sacrifice pay for seniority on a schedule for a majority of the month. Let the union vote on it as an option.