But the real scenario is going to be different in two ways: Market capitalization of the new company will only be a small fraction of the index total, even after it's been inflated as indicated. And not all investors in companies on the index are index funds, which brings down the number shares needed to align a fund.

Maybe they propose the rule change because it adjusts for some other problematic effect of the existing index rules? The discontinuity might seem acceptable because it is unlikely to be reached according to their simulations.