Or, beyond a certain size, mass, density, the physics of the Universe could have them transform into something entirely different, such as an anti-blackhole.
Anti-matter in a particle accelerator, as I see it, is a by product of smashing atoms. Disregarding the infinite of the Big Crunch, big enough, the crunch of a blackhole equals the smashing in a particle accelerator probably creating an entire anti-matter mirror blackhole. It is the last straw, so to speak. It is that big. It is that powerful. It is that kind of crunch. The mirror creation tries to occupy the same space at the same time as the original and they are then both gone in the energy of mutual annihilation. What is the winner? The universe. Balance.
"Anti" is not a "mirror Universe" that once existed with the Universe supposedly winning an Armageddon-like event of annihilation. I see it existing as a ready made universal instrument of balance. It co-exists within every universe and existence ready for every imbalance having reached a point too far. A "SLAB" is likely one of those points (eventualities) having reached too far (having crossed the line). Its "SLAB" mirror self will, of course, have crossed that same line, reached exactly that same point too far, at exactly the same time. Now pointed straight at mutual annihilation.