It's viable. It's just not optimal. Most groups progressing in Final Coil are not Lucrezia or DnT. They're going to be significantly better geared by the time they get to the later turns eventually. So the disparity between a good DRG and a good MNK, while it'll still be there, won't be a big deal. MNK+MNK is/was one of the best things you could bring DPS-wise and it didn't really make everyone bring it to everything in pickups (partially for loot reasons I imagine) or statics.