It's a nice idea in theory, but the problem is that only one side has actual control/ownership of the domain at any one time.

If the startup owner gets control, they can decide to stop paying, and the original domain owner has to sue for their money.

If the original domain owner keeps control, and the startup does well, they can be held to ransom by the domain owner threatening to redirect the domain.

Of course in theory all of this can be prevented through contracts, but the prospect of having to use the (international) court system to enforce one's rights is not an attractive one for either side. Startups often run out of money, and domainers like to spot an opportunity, so there's just too much risk.

As an aside, names aren't actually that important, so long as you can own your namespace. Flickr? Facebook? Craigslist? These are all pretty bad names, but it didn't seem to matter.