Nothing stops parents from passing along their fandoms...except the parents not having kids. I am of the belief that semi-active Sailor Moon fans as people are much less likely to have kids to begin with.
Some of this is due to demographics, since in general people are having fewer children. You are aware of the situation in Japan. Western Europe is in a very similar situation and even North America is not immune. Dual Income No Kids is becoming a norm, not to mention the rising rate of singledom. It's hard to grow an audience when the population is either slowing in growth or on the verge of collapse.
There's also the self-selection factor of Sailor Moon fans. Sailor Moon fans tend to be more progressive, and progressives tend to have fewer kids. (Note, I'm not saying there aren't more conservative Sailor Moon fans, but that the people who still love the series lean as a whole towards the left).
The biggest factor, though, is that there is no new Sailor Moon material aimed at kids or teens with the possible exception of the musicals. Thus the people who are already fans invest more into it, and the franchise in return caters to these fans.
As a general rule, you need to be able to reach kids if you want a franchise to last generations, and the only way to reach kids is to produce material aimed at this group. This means you have to start from scratch with a new adaptation. Reprinting the manga is not going to bring in that many more new fans.
Plus, while there is an intergenerational appeal for many megapopular franchises, Sailor Moon isn't one of them. Instead the franchise gets its money from an aging collectible crowd who likes to spend money on the franchise, making it a very circular relationship. Everything about Sailor Moon is marketed towards older fans and nostalgia, and now even Millennials are starting to age out of the coveted demographics. By marketing Sailor Moon as a nostalgia product, it displaces all of those who don't have nostalgia for the product. My parents both loved The Mod Squad, but I personally don't have any love for it.
I'm not saying it can't change. Sailor Moon has all the ingredients for a true revival. However, the series itself would have to change. Right now, despite the Crystal movies, the musicals, and the novelization, this franchise is still preaching to the choir.