All this talk about cutting rewards for losers in FL or not, about freeloaders for roulettes, people throwing on purpose, etc, made me think that you'll probably never solve this conundrum fully: adding rewards will essentially draw in people from other parts of the MMO that just want said rewards but hate the mode or just don't want to engage enough with it, and will also encourage people that actually enjoy the mode to "optimize" their investment to get to said rewards better or faster.
This is not a hazard that in current FL a lot of the population is just there to level up their jobs. It's not completely binary and black and white, some may hate the mode, some may like it, some may be okay with it and the xp is the motivator that makes them queue for it either way. Cut this out and FL becomes a dead mode, much like if you cut the crystal series and casual will peter out as well.
This is not a hazard either that the more rewards we have had in the competitive ranked pvp modes in XIV, the more win trading and obnoxious toxic behavior has happened, because rewards were limited in nature. Currently there is a lack of rewards to keep people engaged into ranked which is something that has been universally panned by the ranked pvp crowd, but I truly wonder what would happen were we to get seasonal top 100 mounts and unique gear and whatnot?
I'm not sure there is a true and serious answer there. This is a MMO community and it's inherently driven by rewards. Put this in stark contrast with a lot of multiplayer games where even if the multiplayer population still remains a minority (solo, campaign/story and co-op modes are always more populated than ranked no matter the game), the rewards are generally pathetic if there is even some, yet they do play ranked consistently. It's not the case with XIV, where people don't play the sport for the sport itself fully.