A big problem is that capitals are somewhat balanced around a low fleet cap to help make them efficient by the fleet slot measure, especially the logistics capitals. When you consider their role and impact on fleet speed, fuel, and sensor profile, anyway. No slot cap also makes weird 12 tug 20 base burn fleets possible uncomfortably feasible. I mean it's already possible but you'd eat up half your slots with tugs and Prometheuses to carry all the fuel which hurts it a lot more than its raw fuel cost does.
This could obviously be fixed by just improving the efficiency of capitals and making tugs more expensive though.