Here’s the problem: if you mandate that you get enough inflation that our Universe can exist with the properties we see, then outside of the region where inflation ends, inflation will continue. If you ask, “what is the relative size of those regions,” you find that if you want the regions where inflation ends to be big enough to be consistent with observations, then the regions where it doesn’t end are exponentially larger, and the disparity gets worse as time goes on. Even if there are an infinite number of regions where inflation ends, there will be a larger infinity of regions where it persists. Moreover, the various regions where it ends — where hot Big Bangs occur — will all be causally disconnected, separated by more regions of inflating space.

Put simply, if each hot Big Bang occurs in a “bubble” Universe, then the bubbles simply don’t collide. What we wind up with is a larger and larger number of disconnected bubbles as time goes on, all separated by an eternally inflating space.