Oh my God Kanako, I just thought of something else!
We all know how Maribel dreams of a world where magic and monsters exist called Gensokyo, correct? Maybe the truth is that all of Genoskyo is nothing more than a dream.
Think of it. In this day and age you don't believe in monsters and magic right? Therefore not believing in them means they don't exist. Such beliefs are "dead". However, the one place where they can exist is within someone's own fantasy. Their dreams.
And Reimu herself has those as her signs. Fantasy Sign and Dream Sign. The boundary she guards is probably the boundary that separates the real world from the dream world. Dreams cannot interact with the real world, in which case they are just nothing but fantasies; things that don't exist. It could be that Gensokyo itself is simply a metaphoric world of fantasies that still exist within our imaginations. Never dying and everlasting. As for Yukari seemingly passing through the barrier between Gensokyo and the outside world, maybe that too is a metaphor. For what? ...I have no idea. Fantasies becoming reality by perseverance? I don't know. Maybe it's magic. :V
Of course, one might also bring up Sanae's entrance into this "world of dreams". I could toss in a ton of random, nonsensical theories that won't make sense. But...they won't make sense. Perhaps Sanae is just another entity in this dream world. Perhaps she's not really from the real world but an embodiment of the mind that still clings to fantasy (which would explain the goddesses that accompany here), so she truly fits in Gensokyo. I'd say this applies to anything/anyone else that isn't native to Gensokyo.
Take it or leave it, but I feel that it makes sense to an extent (key phrase).