Twitter had plenty of competition. All existing social media was competition for Twitter. Email was competition for Twitter.
Competition isn't about some other founder doing the same thing you're doing. It's about all the ways people can already do the things you're trying to sell them.
That is my point - nobody was doing microblogging before Twitter, they literally invented new mode of communication. Yes, it could have failed but not because email or social media would steel their users.
What does such investor expects to hear in if you are Twitter or similar company entering a blue ocean (creating your own market)?