If there are 'only' 5-6 startup hubs today, not too long ago there was only one. Since the other 4-5 hubs managed to grow in spite of SV's existence (and dominance), I do expect this number to increase further, by one order of magnitude at least.
I don't think the star-system is necessarily a relevant analogy because it is focusing on individuals, who as actors, actually have quite a few more options than just Hollywood. Rather, what would happen if 50% of the next generation of movie producers and production companies were from St. Louis (or Bollywood)? Or, for a different scenario, 50% of the next generation of SFX and post-production companies? Do they all have to move to Hollywood? Can some of them move back? For some types of companies, It's actually demonstrable that they don't all make the move, and a rash of soundstages and post-production facilities are sprouting up in secondary hubs across the US because Hollywood is too expensive and crowded.
Your empirical argument is well taken. By no means was I arguing that SV would become irrelevant to the current industry because of growth per-se, but you're assuming that startups are (and will remain) an 'industry', and second, that they will remain the same industry.
Personally, I anticipate quite a bit more disruption and creative destruction than that. The US rust-belt is testimony that further growth in an industry can under the right circumstances pass existing hubs by, eventually making them irrelevant because the industry itself has changed in fundamental ways. It is also possible that 'startups' might get distributed into the broader economic fabric as thoroughly as 'web design' or 'desktop publishing'. This may actually be required if companies in all industries (in industry-specific hubs everywhere) are to grow by acquiring startups rather than hiring.
If the trend if that many more small startups with bold ideas start to win, other VCs have to become more bold, and so the difference between, for example, the Valley with their aggressive VCs and other areas would diminish.
I do agree with your basic premise though.
How about textiles, after the spinning wheel came to Europe and the cottage industry developed?
And in any case I doubt the number would increase. The power of hubs is self-perpetuating. So no matter how many startups originated in St. Louis, there wouldn't be a lot there when a given new one started, because many of the previous ones would have already moved.
(Suppose it happened that 50% of the next generation of big movie stars were from St. Louis. That would not make St. Louis a rival to Hollywood.)
Also there's the empirical argument: I can't think of one industry in history whose growth made the established centers irrelevant.