How does total number of employees is a good proxy for success of the company?

In my opinion, total number of employees is a bad proxy as it fundamentally masquerades the unit economics and profit margins once they have revenue in several multiples of millions (which is what founders should have on their mind all the time if growth is healthy). Also, naturally logistics related startups will have more full time employees vs. pure SaaS company. Venture business is run by outliers. And 2 of the outliers of last 5-6 years were Instagram and WhatsApp - both of which had less than 50 employees at the time of sale.

Would love to hear thoughts of YC on this.