Indeed you're right that only Yuzu has an exceptional fanbase. I think, it is a longing for a "bright personality in the history" in the dreary postmodern age. And can you imagine how powerful that is? But it is not something that we need to be afraid of, this should rejoice. Because this can be used for the benefit of our favorite sport.
I believe this power really should be used to advance figure skating interests.
I think, people do not gloat over the ISU but hope for reforms. And if anyone has the this opportunity - it is Yuzuru. Because he has a lot of support, resolve and will.
But let's see what the future will hold.
I love your optimism, but the main problem is that the vast majority of the fanbase are convinced that the ISU was/is incontrovertibly "against him". As such, the more fervent part of the fandom doesn't consider someone a 'true' Hanyu fan unless they are expressly and openly
against the ISU. The only acceptable views of the ISU is derision, cynicism, or even anger (although this has got to the point that extreme devotees have held up protest signs or even defaced arenas); any show of support towards the ISU will have you ostracized by the fandom's community. Thus, most of this fanbase will never support the ISU -- and by extension figure skating -- now that Hanyu is retired.
To many of them, the ISU is just some monster who was merely 'using' Hanyu to fill seats and now in his retirement he's "free of them" (lol, as if he couldn't have retired whenever he wanted to). Not sure what the ISU should have done - should they have compelled the judges to give Hanyu/ the same lofty scores his own fans felt he was entitled to (even when Hanyu
won people griped he should have been scored higher), while dropping the scores of his rivals? Should the ISU have implemented rules designed to maximize Hanyu's scoring potential (e.g. the base value of Hanyu's fairly consistent quad loop should have been boosted to be worth more than the base value of the quad lutz/quad flip -- which coincidently happen to be much less consistent/non-existent for Hanyu but are consistent jumps for his rivals; the quad axel - which Hanyu or anyone else haven't even landed yet - wasn't worth enough points and the stingy base value was intended to dissuade Hanyu from attempting it, etc.)? Should the ISU have featured Hanyu more prominently in their communications/events, instead of sharing the love with other skaters and disciplines who were bringing in comparatively less revenue?
As karne mentioned, most of them aren't primed to support the ISU, but in fact, the other way around -- they are eager to see the ISU crumble to "prove" that the ISU needed Hanyu and not the other way around. But let's not get it twisted -- if not for the ISU making figure skating happen, Hanyu (or any elite skater) would not have the career/relevancy/popularity that they've had, and that's a fact. The ISU existed long before Hanyu and will continue to exist after him, regardless of whether or not they have the subsequent support of his fans.