It has felt for a while that the valuations of private companies had soared to unsustainable levels. It seems that the correction is underway. The people are starting to see that the Emperor is, in fact, naked. 

I recall the 'RIP Good Times' presentation by Sequoia Capital in 2008. It marked the start of the last correction. 8 years before this the 'Dotcom bubble' burst. Now, 8 years on - a pattern is emerging. 

That being said, whilst undoubtedly overvalued, the vast majority of these businesses remain strong. They are growing, and they are - in many cases - highly profitable, or can become so very quickly if they need to. Valuations will always ebb and flow but I suspect the vast majority of these companies are here to stay. There will be some difficult times, but they will emerge as strong businesses - disruptive forces in their sector - many of them with justified $multi-billion valuations. In the meantime though Founders will stop talking their valuations up, and stop accepting onerous terms from investors to justify a $billion valuation. 

That can only be a good thing. What is required is less unicorns, and more stallions.