Exactly, or a company insider with options, preferably an early-stage employee. Being an accredited investor or early employee in a successful company dramatically lowers the pool of potential candidates to strike it rich.

You could still make a really nice return post-IPO, but it means you need a certain minimum amount of wealth you can risk investing to begin with (i.e. being able to access say, $100k or $1m and hope for a 10x return. This implies you are already upper-middle class or wealthier). I made up figures here so that the return would be life changing for most people. You could hope to turn $1000 into $10k but it wouldn't be that life changing for many people. Again, being able to access, say, $100k in liquid assets would dramatically lower the pool of candidates. Bitcoin was likely a one off like you say.

I disagree. No more than getting in as an early employee at Microsoft or Facebook or Google, or in on their IPOs, or even in early after the IPOs.

Apple returned 100x between 2002 and 2012. Tesla is up 300% the past 12 months. I've seen penny stocks go up 1000% in a day.

It's not like Bitcoin is perpetually turning out millionaires; the money has been made by the early adopters. It's not really any different than any other speculative venture.