Okay, first of all, it's people like you that are killing my faith in this community. People who don't really understand the balance of the game, and literally make stuff up out of thin air.
BRD literally never got nerfed. Their potencies were never touched. You are spreading misinformation (additionally, the two-bard comp was really only made popular because of T2 silencing, not because of superior DPS).
Cross-class got nerfed. This affected all DPS, just BRD moreso than others, due to cross-classing both Blood for Blood and Internal Release (which got the un-traited buffs cut in half), compared to MNK and DRG only cross-classing one. SMN was hit harder with the loss of Thunder cross-class entirely. Well, back then at least.
You also clearly have no idea how hard -20% (now -15%) damage hurts if you are constantly rotating songs. To reiterate, BRD did not get nerfed, and if they spent/spend the whole fight singing MP/TP songs for "infinite tp and mp," their damage would ridiculously gimped and they wouldn't come close to Caster DPS.
Now, that said, WM and GB are certainly not restoring any "lost damage" since your nerf story is completely made up. It should absolutely be a buff that allows them to take a pure dps ranged spot, because the buff should be significant enough that turning it off for mobility should be enough of a DPS loss that they won't compete with a Caster, forcing them to learn the ABCs (always be casting) of dodging efficiently like BLM has.
And their support should have no affect on this. If you want to take a pure ranged spot in a group, chances are you don't want to be providing tp/mp support anyway, but just to clarify, since I guess no one knows this, providing TP/MP support already lowers your DPS! No need for any further penalties! Isn't that great? Now everyone can stop complaining about it. And if we want to start comparing Requiem and Hypercharge to everything NIN and DRG brings for party DPS, well... lol.
Allowing WM and GB to bring BRDs and MCHs up to competitive DPS will not cause any dreaded job stacking, and you are simply fear mongering. One BRD provides quite enough support as it is. No one is going to say "NOPE, NO CASTERS MOAR SUPPORT" if just WM and GB allow this. It would only be potentially overpowered if they were competitive and still completely mobile. Now, will it allow for groups to *possibly* take a 2nd physical ranged instead of a caster, but as a pure DPS that provides no support and is a permanent turret? Sure! And that's absolutely not a problem, no one would feel forced to, and people that want to play a physical ranged without being shoehorned into a terrible support role get what they want. Everyone wins!