There's a really good general question here about how forks of existing cryptocurrencies affect the value of the primary / original chain.
Bitcoin has been forked numerous times and those forks have been forked themselves. I can think of numerous potential effects of a new fork, although determining empirically which effect dominates would not be an easy task.
For example there could be:
(a) A negative impact caused by a drain of investment and funds from the original currency.
(b) A positive impact from increasing the credibility and awareness of the original blockchain if many other chains are founded from it. Basically a marketing impact from reaching new audiences which may find their way back to the original chain.
(c) A positive impact caused by potential improvements to the technology which could feed back into the original chain.
Looking at Steem forks, both the Golos and Scorum blockchains are based on the Steem structure and graphene technology. Yoyow is another graphene social blockchain. The market-caps are currently Golos $7m and Yoyow $4m with Scorum raising $5m through the ICO, compared to $250m+ for Steem.
So the amount of investment drain from (a) does not appear to be huge, in particular since much of this investment could have come from their particular specialised communities (Russia, China, and Sports) rather than from investors looking at Steem.
My guess is that the positive effects of b and c from multiple adoptions of graphene technology could end up being more valuable than the investment drain.