I don't know if this will be any help to you but ... my LFS
has had a Moorish Idol for almost three years (they got it from a guy who had it for not quite two years). It is doing great BUT they have it in an around 2000 gln. tank and have had over fifteen years of experience with harder to keep fish (which, by the way, they refuse to order or sell any of those fish - which I see as a really good thing).
I have never heard, personally, of anyone else ever keeping a Moorish Idol alive for more than a couple of months.