Coinbase.com recently updated their site to reflect 10 Million users:
http://www.zerohedge.com/news/2017-09-05/us-bitcoin-exchange-coinbase-hits-10-million-users
That's a big number, with a lot of buying power. What if every single user decided to set their account to auto-purchase $100/worth of Bitcoin every month for the next 2 years, and hold for 3 years? Even if even some users purchased less, the average could easily be $100.
What would happen is, there would be a Bitcoin Shortage! At today's price, that would remove 4-5 million Bitcoins from the float. Take into account that most experts believe about 1/4 of bitcoins have been lost forever:
https://seekingalpha.com/article/4082979-many-bitcoins-lost-forever
And between Coinbase and lost coins, 1/2 of all Bitcoin would be accounted for.
So how high can Coinbase users take Bitcoin? $10k would not be out of the question.