darkcoin brought new people into the cryptosphere
How many?
Compare with Steem.
If the ratio is less than 100:1 I'd be surprised, and will likely very rapidly expand to 1000:1 or 10000:1 if not higher.
What is Steem's attrition rate on new signups? Since no one is compiling this data accurately, we don't know if Steem is failing or not.
I don't know the number, but regardless of attrition rate it there are still orders of magnitude more people actively using it than were brought into the cryptosphere by darkcoin. Come on, it isn't even close. Just look at the volume of substantive comments being posted which is sort of a Proof of Actual (non-attritted) User. Unless you think Ned has hired an army of sock puppets in the Philippines to pretend to be users. That is possible, of course, but I just don't believe it.
That doesn't mean a competitor to Steem couldn't do even better.
no forking is allowed so I'd have to start from scratch
Not entirely from scratch. The Graphine code base is fully open source (MIT license) so you could start with that.