The following quote from your post points to one of key reasons I see decentralized social media like Steemit succeeding:

• "Now, what happens when unlimited likes become limited investments? People become a little bit more serious with their votes."

People will see true value in the things that are upvoted because up-voters are incentivized to only approve of meaningfully well produced content; Theoretically.

Of course you get problems like up-voting bots intended to maximize curation rewards, but I still think the Steemit approval system is objectively better than the approval system of something like Facebook. The Facebook like, and even their "friend" system are in my mind relatively weightless and therefore meaningless.

Facebook: Friend anyone, like everything.
Steemit: Follow meaningful content creators, upvote great content, downvote bad.