Will be probably happen. I guess the trigger will be something like Twitter going down under and not getting VC the payment that they WILL WANT in some close future. I guess then comes a few years of VC being a lot more cautious where it's going to be harder to sell your company idea to them. It's phases all the time and we're on a high right now.
Although right now money is so cheap and there isn't really any alternative if you want ROI. That said, US interest rates are on the rise more so than those in Europe so perhaps the end might come sooner than most people think.