>how many times have you or someone you know hesitated leaving a current employer due to benefits uncertainties?
Hopefully 0 times. Major changes in your life (like changing jobs, or moving cities, or making large purchases) always require some consideration.
>human ability is not really allocated all that well when you're bonded to an employer without cheaper switching costs.
You're overstating the case especially in market economies. If anything, the issue is more prevalent if your employer is the government, as the government tends to overpay for labour and is slow to adjust if conditions change (i.e. very hard to let people go for any reason).
Having said that, there are problems with movement being too liquid. That is, you lose institutional knowledge if your company (or government department) has a too high of a turn-over rate, which leads to lower quality products/services.
>how many startups never happen because the right people are unable to manage the personal risk of leaving defined employment?
And rightly so. Startups are an incredibly risky endeavour and they always will be. This is why founders tend to be younger, rather than older. If you have dependents and responsibilities you have to think twice about making a high risk bet. I don't know if you want to actually incentivize people to make more high-risk bets.