You missed a few threats:
User adoption falls off as new users realize probability of reward very little without powering up for a significant investment (4 to 5 figures).
User attrition accelerates as Steemians who have been here 2-3 months see their rewards dried up.
HF20 doesn't address the self-voting problem, where all accounts of medium to whale level SP are incentivized to click on their own posts, or their own circle of friends, and the average user needs incredible luck to see any rewards.
Whales start powering down and crashes the price (you had price crash in there but didn't mention how)
Copyright infringement risks causes Steemit to have to devote excessive resources to edit posts / deal with takedown notices
Copyright infrignment liability if sued by major publisher will create a lot of uncertainty and doubt in Steemit's future. For exampe, see https://steemit.com/steemit/@tommymindtrick/the-steemit-sandbox-puts-us-all-at-risk-due-to-ewhoring
Thanks - yes, there are potentially many other points I didn't touch on.
I'm not sure I agree with your first point since I've been getting decent payouts since I only had a few hundred steem and I didn't buy any of it. The site is intended to reward quality content and for solving problems - I think if users actually do that then they can thrive. It's a good challenge and inspiration to be able to test your ideas against a captive audience of readers and thinkers in this way.
I have seen that some of the whales have been powering down for a while, but I'm not sure that has effected the price of steem to a problematic level or not - I appreciate they could all pull out at any moment, but that would equal a general catastrophic loss of confidence in Steemit for some reason that would probably have a different ultimate cause than just their own personal need for cash.
I agree about Copyright and general legal issues - my exploration of this theme so far has only led me to the answer that 'the founders don't really care about the law'. lol.