The most important thing in any story is to have powerful characterization. FiM has this, and that's why people like it. Age is, and always has been, irrelevant. Why do you think Pixar movies are always so good?

True story. I personally enjoy FiM because the characters' personalities are very relatable. Also, there is just so much wiggle room with FiM. The characters are all very unique, and they're all just really interesting to watch in general.

Anyway, I'm really bad at trying to put my thoughts into words, so you'll have to excuse me. I'll definitely give 'Beneath The Ice' a read when I can free up some time.