I agree with you- basing a startup on one of these platforms is incredibly risky (on top of the standard high risk that all startups have).
I myself have written a TON of code for Twitter, for example, that is no longer useful due to ever-increasing restrictions on the Twitter API (combined with the network growing more and more. I want my 20k API requests per hour back!). Since these platforms can change their terms and access at any moment, you could easily have a great idea and implementation that they invalidate accidentally (or intentionally). That's just too much risk for me.
I myself have written a TON of code for Twitter, for example, that is no longer useful due to ever-increasing restrictions on the Twitter API (combined with the network growing more and more. I want my 20k API requests per hour back!). Since these platforms can change their terms and access at any moment, you could easily have a great idea and implementation that they invalidate accidentally (or intentionally). That's just too much risk for me.