Making a market for a mid-point milestone was a good idea.

This market is roughly the halfway point for 500 launches by 2030, on an exponential growth market.

I think that makes it somewhat more likely than the linked market; if SpaceX hits 500 launches by 2030, I strongly expect it will look like an exponential path to get there. But it's also plausible they'll have a quick start and then slow down as they hit later sub-exponential constraints like launch sites, demand, regulation, and so on.