Greater portability could in theory lead to higher turnover even among happy employees. They might go on to found their own company sooner. They might see good financial sense in diversifying their options portfolio. Yet young companies need the team to stick together for a certain time. Especially very small startups at the YC stage -- turnover is very harmful.
Note: In Adam's example, nobody leaves the pre-IPO company in under 4 years of service.[2]
[1] https://github.com/holman/extended-exercise-windows
[2] "imagine a company takes 10 years to IPO. Employee A works at the company from years 0 to 4. Employee B works there from years 4 to 8. Employee C works there from years 8 to 10."