The history of the Old School Runescape Polls is a massive rabbit hole that OP has completely glossed over.

The provided example for a new skill was exactly one poll in the game's history. Almost every other poll is a Yes/No question asking about whether the devs should make a particular change. In XIV's terms, this would be literally every change that is mentioned in the patch notes requiring a 70% Yes vote to be implemented.

Since the polls can be voted on by anyone with a small amount of game time played, changes that would only affect small portions of the playerbase can be affected by players who have never even touched that content. In Runescape's case, this led to years of neglect for PVP, which would almost be a guaranteed victim of this system in XIV. This also means that every adjustment to jobs can be voted on by the entire playerbase.

It is hardly worth elaborating further on the potential flaws of this system because OSRS and XIV are just not remotely similar games. You would have to completely change the content cycles of XIV in order to have players vote on new additions like they do in OSRS. The more important thing to leave with is OP's claim that this system "provides tangible feedback for the developers."

It actually does not. This is a system purely meant to cater to that game's target audience who successfully managed to win a legacy version of the game once the original had changed far too much from what they enjoyed about it. The system holds the developers in check from wildly changing the 2007 version of Runescape that is so beloved. They actually receive their feedback by communicating with their community via dev logs, responding to questions on Twitter, and showing that they read their subreddit and forums. (And the only reason this even exists is because the original version of the game: Runescape 3, is a whale farm of microtransactions to generate funds)

If you want a polling system in FFXIV, the best thing that this game could get is just more attention given to feedback questionnaires. As mentioned above, some people get them, some people don't, and some people just have no idea that they exist. They really do need to do a better job of showing that more players than just streamers can have a voice in the direction of the game, and they can do that by treating feedback questionnaires with more care and attention.