You think this time big financial institutions are going to maneuver quickly enough to become viable competitors in this market before a startup establishes a decent foothold with a comfy moat?

That has literally never happened before to my knowledge. See online payments with Paypal, robo-investing with Betterment, crypto-currency with Coinbase, etc.

The more likely scenario would be other startups rising up to compete with them, but it seems like these guys have a decent head start with 100 million funding and what looks like a compelling product judging from their traction.