The content doesn't scale because 75 was supposed to be the cap forever. If SE had intended to go beyond that(notice how foresight failure is becoming a theme here) then characters and mobs would have increased in power faster than they did. Instead, there is about the same difference in power between a level 50 and 55 character as there is between a 70 and 75 character.
It doesn't matter how many people actually have the weapon. It isn't a necessity that you own something to be able to tell how it works in practice. I don't own a Ferrari, but I know they eat up road like nothing else.
SE has decided that their opportunity to scale characters to higher power is going to come in the form of instance specific events like Abyssea. As I've already stated Ukko's being broken wasn't a factor there, but depending on what becomes of endgame in XI, it very well may be broken elsewhere and it happens to be the case in VWNM. It's also completely possible that future gear will ascend warrior close to, if not completely back to the current 80% crit rate for Ukko's.
It remains to be seen, but at least give the 99 cap time to settle in before you rush to dismiss a change that doesn't have that much of an impact overall.