I think you’re asking the wrong question. Steemit is really just an application running on the Steem blockchain. So the real question is can the Steem blockchain scale?
Take a look at http://www.blocktivity.info
The Steem blockchain is already running circles around other blockchains. And doing so running at far less than 1% of its current capacity, let alone any capacity that might be added in the future.
RE: Can Steemit Scale? Will Rival Platforms Appear?