I agree with this 100%; RNG as a reward mechanism is absolutely not a good methodology to maintain, much less attempt to capitalize on in order to garner more play time.
Before I dig into my explanation of why, I feel the need to correct a fallacy I see folks post frequently on these forums:
"RNG is RNG."
No. No it's not. Want to know why? Because technically "random" doesn't exist. It's not some natural phenomenon like water being wet or the rotation of the stars across the sky. Believe it or not, "random" is a perception. Only a perception. With enough data, literally anything can be determined, so anything that appears to be "random" is just perceived that way due to a lack of understanding of all contributing factors.
What then makes this whole "Random Number Generation" so frustrating is that it is 100% controllable, because it's generated via complex algorithms that define what the "random" parameters are supposed to be. Las Vegas is amazing at this because they've learned to walk the fine line between rewarding and punishing that addictive habit of seeking a big win...
Which leads to why I don't like RNG at all: It's a mechanic specifically designed to push players toward repeat playing (which isn't bad unto itself), but can quickly become a predatory tool depending on how sever the "randomness" is set to. And in the last several years, it seems more and more games are hoping to capitalize on "randomness" to push player interaction.
Now this game is better than most, I'll admit that, but I've already seen a few instances of RNG that are insane, and already has me backing away from repeat playing of some content. For example, I wanted the Fending chestpiece from the Skalla dungeon. Want to guess how many runs it took for me to get it? Yeah, 100+. I literally stopped counting after 100 because it was just asinine to me that it was even taking that long. NOTHING should require that many runs to complete in a video game.
What winds up happening at that point is there isn't a sense of joy upon getting the item, just a sad sense of relief of NOT having to do "that thing" anymore, which then leads to saltiness of ever having to repeat that content later due to DF runs.
Especially when this game already has a lot of much, much better systems in place. In some ways the Duty Finder is an excellent example of how rewarding the players in different ways to run old content can be extremely beneficial, as it allows players to partake in a variety of content to work toward an end goal, rather than having to grind their soul against the edge of some random chance of hope. Working towards acquiring items, tomes, etc. is already done in a lot of places and while sometimes the grinds can be hefty (I'm looking at you, Resistance weapon), at least that still felt satisfying upon completion!
Although there is still the issue of some players already having completed said content many, many times before, likely due to RNG hunting so they're now burnt out on running that content again, but I addressed that earlier already, lol
Either way, once again I have to agree with the OP; RNG is not a good "entertainment" mechanic to maintain in a video game. This game is already better than most by being thoughtful of how it affects its players, so I hope consideration is taken to tone down further uses of RNG and maybe even someday remove it altogether as we already know there are much nicer ways to maintain player interest and retention than through cheap "random" tactics.