Not sure how crazy you think whales are, but typically, people stop pulling once they get the limited kami (since that's likely why they were pulling in the first place), so it's very rare that they end up with multiple copies of said weapons (unless, by some miracle, they pull 2 in the same pull). Normal kami weapons, on the other hand, you can definitely get dupes of if you pull a lot (even non-whales often end up with a dupe or 2 just from jewels). This is why normal Kami weapons has FLB, and not limited ones, just due to accessibility. That said, maybe in a few years, when they come back enough times, ppl might run into that problem and they'll also get a FLB.
Epics/Raid weapons getting FLB honestly seems pretty unlikely, the reason Advent weapons gets FLB is because they come back as actual (filler) events, and they want people to farm it despite being a repeat. Same for UE weapons. Epics just doesn't have that incentive, so it's hard to see them putting in the extra work to making them good (because it doesn't matter to them if you farm it or not). They seem to be there just to help new players fill out their grid a bit easier. That said, the current system seems pretty lack luster so far, so maybe they'll end up changing it later.