Using your metaphor, this rule change represents something more along the lines of an invasive predator, new disease or parasite.
I'll stop there, because Musk is much-loved by the internet, but to pull on this string of systemic risk, let me see if I can walk through my thoughts here:
Nasdaq Inc. (the exchange operator) and Nasdaq-100 (the index) are not independent entities with separate interests. Nasdaq owns and operates the index. A SpaceX listing would be a major win for Nasdaq, reinforcing its dominance in IPOs and billions in licensing fees - Invesco QQQ Trust alone has over $300 billion. Every basis point of licensing fee on that is enormous static income. When SpaceX says "change the rules for me or I list on NYSE," that's a threat to both the exchange listing fees and the downstream index licensing revenue.
So the competition here is between index operators (SP500 is actively managed by an actual steering group, so rules changes aren't as big a whoop) for who can net the biggest pool of new shares so that their passive income on index license fees can get maximally farmed. The actual thing the money comes from - retail index investors - don't figure in this equation at all. It's a systemic risk of replacing actively managed funds with consumer 401k products, risk that requires active legislation and law enforcement for there to even be a fair marketplace; markets require regulations, after all[1].
If you're thinking this sounds a little like the competition between the ratings agencies circa 2008 to see who can award the most AAA ratings, you're in the right ballpark. It wouldn't even be possible if SpaceX wasn't onboarding basically every big player in this travesty: Goldman Sachs, JPMorgan Chase, Bank of America, Morgan Stanley, and Citigroup. In fact, I suspect that this cabal of villains are the ones testing the x{N} float idea that has everyone up in arms - they see a golden plan here to smash the window and hit the road before the sirens go off.
And that's the problem with invasives. They don't participate in the system at hand; they monkey with the fundamentals. Compete with other plants? Nah, I poison all the soil where I grow. Not enough substrate? Kill all the vertebrates with symbiont viruses so the world turns into Fungusland. And even if the retail investor wins, that means a massive flight from NASDAQ, which even I have been double checking in my tiny little pool of casino winnings. But triggering a flight from every index everywhere? That would not be bright. But, well, that's the other thing about invasives, right? They're not long term thinkers.
[1] I suppose this has become a political point too, hasn't it? Well, ride with me for a bit.