The public market isn't really expanding and has essentially become zero-sum. This doesn't preclude the existence of other capital markets that might actually be taking share away from publicly-traded companies - for example, late-stage VC/PE financing (a la Uber, Lyft, and most other Silicon Valley unicorns), crowdfunding, or cryptocurrency ICOs & STOs.

Nature usually abhors a steady-state: when you think you've reached one, it often means that there's a competitor that's too small for you to see but growing exponentially.