(Heartfelt thanks to 'Lei Xiang,' 'Wu Feng Cao,' 'Yuyuko's Lament,' 'Wu Feng Cao,' and 'otakujerry' for their rewards!)
The so-called Great Hakurei Barrier was, in fact, a barrier that distinguished between "common sense" and "the extraordinary."
Under the effects of the Great Hakurei Barrier, it automatically differentiated between the "common sense" and "the extraordinary" of the outside world and Gensokyo. What was common sense in the outside world was extraordinary in Gensokyo, while what was extraordinary in the outside world was common sense in Gensokyo.
This was not something that existed in a physical sense, but a barrier that existed in an ethical sense.
Moreover, the Great Hakurei Barrier was extremely powerful. Even youkai could not pass through it easily. To this day, apart from Purple, Gensokyo's creator, who possessed the power over boundaries and the ability to use Gaps, no one had been able to freely travel between the outside world and Gensokyo.
At the very least, aside from Purple, no other youkai, fairy, spirit, or even deity in Gensokyo had tried.
In other words, the Great Hakurei Barrier was the barrier that truly cut off exchanges between the outside world and Gensokyo, giving rise to the place known as Gensokyo.
To put it another way, if Gensokyo were compared to a "house," then the Great Hakurei Barrier would be the "walls" that formed that house, truly isolating its inside from its outside.
The Boundary of Illusion and Reality, meanwhile, was a nonexistent "door" within that "house"—a wondrous door that automatically brought in "residents."
That door had been created by Purple, so as long as she still existed, she could create such a door no matter where she went, bringing in "residents" and turning Gensokyo's "house" into a "home."
But if the "walls" collapsed, then it no longer mattered