With the pilots who are sitting reserve, getting paid as Captains.
The only change would be reserves could aggressively pick up both FO and CA trips, or be assigned such by scheduling, but the window to do so would not change.
Captains can fly in the right seat. Some can even do a good job from the right seat.
I can't remember off the top of my head if its a FOM or UPA restriction, or both. Either one can be changed with the stroke of a pen. I think you also need a right seat check out periodically. I'm not sure if that has to be accomplished in recurrent or as a line check.
At a previous gig a lot our trips required double augmentation. Sometimes (not often) there would be three or four Captains operating the flight.
The biggest drawback I've thought of is for FOs who are too junior to hold holidays or weekends off as a line holder, but can as a reserve. I'm benefiting from that this month, holding Christmas off as a reserve, and I'll go back to being a line holder working weekends in January.
The other downside I forsee is that CAs will sit reserve longer. But that's really a feature, not a bug. As more total CAs will be needed, more junior pilots will have the opportunity to upgrade earlier in their careers, at the cost of sitting reserve longer if they do so as soon as they are able to hold the seat. The number of CA line holders will not change.
What other drawbacks am I missing?
Other than it will cost the company a bit more money. Which ought to inspire them to be more efficient with reserves.