Fatebringer is a special case, like many primary raid weapons. No hand cannon can have Firefly, like no scout rifle can have full auto like Vision of Confluence.
Problem is those weapons will stay at 300.
Yeah; the special cases like FB and VoC are really cool, and it does kinda suck that they're forever 300, but everyone that has a FB uses it in CE. Many are even planning on taking it into HM CE.
A 331 kinetic handcannon seems like it's really purpose-built for PVP, where there are already as-good-or-better exotic options (and if you're a PVP handcannon person, odds are you're not going to be bothered by using your exotic slot for Thorn/Hawkmoon/TLW.
It kinda makes the Lash come off like a charity version of Hawkmoon, so I'm still a little confused about why its quite so coveted.