You're confusing "invincible" and "invulnerable" of having a completely attack preventing base because enemies can not alter terrain to complete path finding or enter/spawn in "dungeons" - with - "circumventing ground based enemies with poor ranged attacks from being able to path/line of sight to your buildings by having shear "cliffs" that the ai pathing is set to not be allowed to climb while still leaving it vulnerable to enemies who don't have to follow ground pathing." The latter is neither invincible nor invulnerable.

Flying enemy invasions exist; we will most likely see more types in the future as well. Most ground based enemies also have a ranged attack. So while the current flying enemies and ranged attacks from ground based enemies we have are fairly weak, that doesn't mean they always will be.