This is a very valuable analysis. Really well done.

Now, each of these - and other social media platforms - could each implement their own cryptocurrency and rewards model similar to what Steemit has done.

This won't be so easy for them. Apart from having a very unique economic model, Steem has a big competitive advantage: a new user can enter the ecosystem without the necessity to buy any tokens. Those platforms you mention would need to have at their disposal a blockchain technology as advanced as Graphene to be able to truly compete with Steem. This won't happen anytime soon.

If there's no "censorship" on porn - are we going to ask investors to accept that it's ok if we allow their funds to support the creation of our digital currency that we don't set restrictions on from being used to support rape, child molestation, or any other form of physical & sexual violence, violation of human rights, or clearly immoral degradation of character?

I disagree with you on porn. What exactly is immoral with porn?

Anything that people produce and are willing to read or watch should be accepted by the Steem blockchain. If Steemit (the website) starts censoring porn (or anything else), somebody will create another Steem-based website which will embrace it. And all those websites will be part of the same ecosystem.

The solution is simple: if you think anything on the Steem blockchain is immoral, just use a user interface that hides this stuff from you. Actually, this situation means that Steem has a very big advantage over legacy systems: as long as they are centralized, they will never be able to offer such a freedom of choice within one ecosystem.