I absolutely understand the people saying the rewards aren't supposed to be the reason why you're doing the roulette, and I agree. I like the roulette for its own sake and I enjoy helping new players out, that's why I stick around always, but the fact is that I'm seeing many cases where players are not. It might be something to do with the differences in culture where this is more of a problem in NA DF than JP, but as it stands, whether or not SE wants players to hunt for that grind or not, the reality is that they are and because players are incentivized to behave poorly, when they do behave poorly because they're only there for the grind, that's a problem with design.
Removing the reward from the roulette and placing it elsewhere would definitely do a lot to cull this kind of behavior, but as others have mentioned, you'd see a huge swathe of the actual potential of mentor roulette to fill low pop content crumble because there's just way fewer people running it. I think mentor roulette has the potential to be an actual positive for new players, but doing that involves actually making it feel rewarding and incentivizing players who aren't naturally going to act altruistically to act that way anyway for some other reason. You can condemn the players who act badly for being selfish all you want, but if you can align their selfish interests with something that helps other players rather than harming their experience, you can get something good out of it.
That's what the talk of incentives is about. It is definitely possible to align players' self interest with helping new players, and this is something which the game already does in other areas. The new player bonus when completing a duty leads to some PF groups being put up specifically advertising the bonus tomes that people will get from helping them out. It's possible to encourage this kind of helpful behavior where you create a win-win scenario between players, rather than as it is, where it's something of a win-lose and you are expecting people to behave honorably. As long as the situation stays that way, it's going to lead to more bad behavior.