I just realized I made a mistake.
An equal-cost force of MBs is in fact helpless against a single, properly equipped TL-15 BB.
I forgot about repulsors. Put enough repulsors on a BB (say, thirty or forty 100-ton bays per 100ktons) and it is almost invulnerable to missile salvos from an equal-cost force. I can put 89 factor-9 repulsor bays on my TL-15 BB, reducing the missile armament to 100 bays in the process (plus losing some other stuff). Doing so means that only an average of 2 missile salvos will get past the repulsors, and then those 2 have to get past the factor-9 nuclear damper, and then they have to penetrate the armor. This is a hopeless situation for the MBs: the BB may only be killing one or two MBs per salvo with its reduced armament but the MBs aren't doing anything at all.
There's also the Damage Control rule. 125 MBs cannot kill BB missile batteries as fast as the 200kton BB can repair them.
Now, to use the Damage Control rule the ship must be in the reserve, which implies that someone else is holding the line, but that's what escorts (or fighters) are for.
Of course the MBs can also use Damage Control and they don't need any escorts to hold the line, but they also don't get the same benefit from it that the BB does, since they only get one DC roll and it only raises their missile rating by one, while the BB restores an entire factor-9 bay to full effect with one successful roll.
As for your MBs against a TL14 BB, a lot more of the MB salvos will hit. But TL14 100-ton repulsors can still guarantee a stop against a TL15 missile battery, and the BB still gets to use Damage Control (if it has escorts). However it's really not practical to put enough 100-ton repulsors on a TL14 BB to stop the kind of missile salvos the MBs can throw, and even trying to do it will weaken the BB in enough other ways to make it more vulnerable.
I haven't completed the math yet, but I'd imagine that an equal-cost force of TL15 MBs could defeat a TL14 BB, probably with fairly low losses.
But a repulsor-armed TL15 BB can laugh at them.