kay, after 4 capitals you have 26 fleet points left...
4 capitals and 26 phaetons (drum is strictly inferior to phaeton) can perfectly travel around just fine.
Prometheus is for when you want to drag 4 capitals across the sector to a remnant system. The fleet caps mean drams literally can't do that. If you want to use a fleet of 20ish ships with capital...
Literally the next sentence established what anyone who has played the late game knows, that end game fleets typically have 20+ ships with multiple capitals consuming hundreds of fuel per light year. You typically have 10 or fewer fleet slots available for logistics ships and you also want fleet slots available for salvaging new ships and cargo haulers. In this case you have 3-4 slots at most available for fuel tankers.
Think about it this way, there are 4 factors that IMO affect which tanker you will buy:
price, supply efficiency, fleet slot efficiency and burn speed.
For the beginning of the game, drams are cheap and fast meaning you can actually afford them and have more money left over for combat ships, and you can run away from all the threats in the game easily. These are your main concerns in the early game.
In the mid game, phaetons are more supply efficient, and more fleet slot efficient than the dram which are the new major concerns in the mid game. Supplies are a very large concern in the middle of the game when you don't really have an income, and fleet slot efficiency starts to be important as you are expanding your fleet with lots of destroyers and some cruisers, approaching the fleet cap. Burn speed is still important to avoid the largest fleets but less important now.
In the late game, supplies, price and burn are all totally irrelevant. You have insane amounts of money from colonies so you can buy however many supplies you want and however many ships you want, and there are no threats outside of stations so burn speed is just convenience. The only real concern is fleet slot efficiency. At this point in the game, you likely have 2 or 3 full fleets of additional ships sitting in storage. You want to bring large fleets to deal with stations and raids and that means you are left with a small number of slots to supply a large number of fuel intensive ships. Prometheus is the only choice in the situation.