Yeah, but user might already be on his way out - not seeing what he was looking for - without making any interaction with website.

The only way I can see it work, is to show scrollbar on every mouse movement. But than I don't see any benefit of hiding it in first place.

However I agree with you that we will see how it works out, when people start using it.

The worst case scenario is probably increased bounce rate on some websites, decreased conversion rate, and some confusion for users.