Renting software for a product like SQL could be a wonderful thing. If you look at Office 365, this is already in existence. You pay a fixed fee every month, and you get the latest copy of Office. If a new version comes out, you can download and upgrade your installed software as and when you like.

I'd certainly expect, with SQL Server, that an initial fee would be required, and likely a minimum term contract, but knowing that when a new version comes out you know you will gain access to it could be a major selling point for many clients.