While too much RNG can be bad, and what Kiara posted is going in the right direction to alleviate some stress of RNG (though the % adjustments might be high in what was suggested), pure RNG formats are necessary for an MMORPG. Players, including you it seems, don't understand the relevance behind it. If every single thing about the game can be planned ahead, any sense of "random" or "RNG" becomes irrelevant. Imagine if you had to do things like the books for Animus on every single facet of the game. You know how boring that is? LOL.
Do you really believe people would enjoy the grind more if they knew they would get said item at this exact moment latest? It's literally no different than saving up gil, MGP, tomes, etc to buy something, besides the fact there's multiple ways to obtain currency. Yay I bought this thing... now to just completely ignore the fact there was very minimal joy in obtaining it. During, at least, the earlier onsets of content, RNG plays the role of prestige and value. Do you really think people would care if Fenrir were an easy to obtain mount? It'd be treated no different than the Behemoth mount (or any other easily obtainable thing). Winning that lotto (or putting in the absurd amount of dedication to get 1mil in such a short span of time so far) is purely for the sense of being unique. RNG (or that diehard dedication) makes you stand out above the rest for a little while.
Again, RNG is pointless if you can predict a way to beat it (e.g. being guaranteed a reward after X amount of time). Thus saying, if it's pointless, then there's no need for it... which means it's no longer an MMORPG (or an RPG in general). They could always remove the boredom of the books by making it faster to complete (i.e. less of a grind), but then things become faster to obtain. Then what? Bored to death due to nothing to accomplish. Not to mention that you'd be raging if you did have the worst of luck in getting to your guaranteed reward (which means you're raging in a similar fashion to how it is now). Think about how it is with people bitching about speed runs or instances taking too long to finish. You already anticipate how long something should take... but oops, a wipe or group aren't moving as fast as you like, so /ragequit. Trust me, it doesn't matter what system they choose to go with... the same problems will still be there. It happens Every. Single. Time. It's just a different target to complain about.