Following your model, then 'innovation' at the big five will slow down and there will be greater incentive for people to create new start-ups... and the cycle will continue.
"It's different this time", but, it's different this time. As the Information Age advances, entrenched players can use their information advantage to outcompete. For example, Facebook has analytics on every small startup that gains traction, and can due much better due diligence than the startup's own investors, and so can buy the most promising ones out for cheap.
The companies still have to be willing to sell, and there's greater and greater liquidity in the market. While the ICO fad will fade, there will be new financial innovations to take the place of ICOs.