Amazon Prime is not a good comparison, people who go on amazon go there to spend money so obviously it's gonna be easier to have them spend money on anything when they have he card in hand already.
Steemit wants to be a social blogging site, social media site are free to use. If steemit starts charging people they start with a huge disadvantage. Steemit has potential because it pays users, something no other social media site does.

Short term speculators

You seem to have an issue with speculators but you havn't explained why you think they are bad for steemit. You mentionned pump and dump but I don't see how this is bad for steemit. Traders do their business and content creator do their. Both can coexist very well.

No Bitcoin, no shadow economy

This is so 2012, bitcoin is evolving. I think you havn't been paying attention but circle has partnered apple imessage, bitpay partner with intel, an ETF is coming soon, many big company accept bitcoin, if you think bitcoin is still shadow you've been living under a rock man.

Mainstream investors have no interest in crypto. So no matter how much you market, you will never draw them in.

Mainstream investors are going to ask themselves how is this platform sustainable and they will quickly come to the conclusion that in its current set up it is not.
Also good luck trying to convince mainstream investors to buy bitcoin, then buy steem, then power up and lock their investment for 2 year. This my friend aint gonna happen.