I'll bring another IP into this just for the sake of discussion. Harry Potter. Their solution to that one was very unique. Grow the characters as the audience grows. It was pretty brilliant, and it worked flawlessly. And now there's new films coming out that are drawing in new fans and... hey look at that... they're not alienating the old ones. Yeah, that's a harder sell for something as fluffy as MLP, but one of the things that makes the show unique are the more serious undertones laying just beneath the surface. Also, the Mane 6 are much more grown up from the very beginning than characters in most shows targeted at little girls. Most shows for little girls have a cast of... well... little girls. The ponies are already matured. They hold jobs. The youngest at the beginning is arguably Twilight, a college student.
Yeah, I get that it's a business. I just see others having little trouble sustaining multi-generational IP's. Maybe Hasbro simply has no interest in being that kind of a company, that caters to more than the K-8 demographic (except when it happens marvelously by accident as it did with Ponies G4). With the merchandise, they barely capitalized on Brony culture as much as they could have. Take Nintendo for example. You can find the Triforce or any other Hylian emblem on pretty much ANY product you want, ranging from t-shirts to wallets. If you want the Equestria "2 sisters" flag on ANYTHING, head over to Etsy. Hay, enough people would have shelled out HUNDREDS for an "authentic" full-size gonfalon on real canvas, supported by poles and ropes. Hasbro could have made FAR more money off of FiM than they did, but they mostly ignored the adults... the people who are just as, if not even MORE zealous than the kids, AND have a lot more money to spend.
"Cartoons are for children" is an outdated concept and some companies still appear to be struggling with that concept. All of Japan never even had that concept to begin with, but the U.S.A. has been particularly stubborn with getting over that hurdle. We're kind of halfway through destroying that stereotype. Disney and Dreamworks sure have it figured out, and I'm pretty sure Disney realized how powerful animation was, way back when with Snow White. For all of Disney's blunders which I could write a whole book about if I had the time, they have a firm understanding of their adult fanbase and give them just as much attention as the children. I get the feeling that Hasbro hasn't figured this out yet; they don't seem to know what they have. The big exception to the kiddies-only attitude might be Nerf, with the Rival line of weaponry... wannabe "airsoft" guns clearly geared towards older players, and not really suitable for children.