The thing about Musing.io is that it is one of the many platforms and applications built on top of the Steem blockchain. The more Steem grows the more Musing grows, and as we all know Steem has so many potentials to go big with all the dapps and platforms being built on top of it almost weekly.
If ever Quora releases it's own ICO, that token will only be used solely for Quora. Sure Quora has many users as of now compared to Musing, but when Steem starts to gain momentum and starts to gain mass adoption because of how apps are interconnected to each other through the power of blockchain, users can just easily go back and forth between apps and platforms including Musing.
So yeah, Musing will thrive even if Quora came out with its own ICO.