Best in Slot implies that there is a best for a specific gear slot. If your gear has randomized substats, then there is no single best piece that can be identified for any slot. The fact that some substats are favored over others is beyond the scope of that thread and I'd love for SE to flesh out the battle system and bring back something meaningful, but even with what we have today certain jobs will favor certain substats over others, and that's part of the game.

If the worry is that the rng formula can be identified in order to find the total stat cap and then the "best item wit the correct combination of substats" that could be generated, then I think that's fair, but on the other hand, if the chances of seeing it drop is one out of millions, then it's as if there was no best. It's a mathematical asymptote: for a single gear slot, the better the gear you find for it, probably after many successive replacements, then the less likely you're to find a better, suitable replacement. To top it of, the way substat tiering works in XIV means that some stats are wasted if not reaching the next threshold, so your very best, unicorn item, may not prove better than something still exceptional, but a bit more likely. And if some people really feel like they do need an unrealistic unicorn in every slot that will give them a 0.01% edge or whatever, and burn their lives into playing the lottery, then they can be my guest if that's what floats their boat. If I get something good in most slots I'd be happy already. Do you think people never stop playing games like borderlands until the mathematically perfect weapon finally drops? Even though they'll have already probably seen a hundred outstanding weapons they could be happy with already? The big idea is that if people feel like they want to see if they can get something better, they can and play the game to their fill, and if they decide that they're happy with what they have, they can also stop. That's what happened with Pyros weapon rng substats (not saying that pyros system was great though). Do you know anyone that got the mathematical best out of this? People are ultimately happy when they get a great result.

And then if we get more exotic effects on some items additionally, then it makes substats even more of an afterthought as long as the base amount is decent/solid. You could even get items with crazy spicy effects, but poor substats.