Good questions. I think ultimately, the answer is going to be the development team's expertise and the project needs that will decide that. ASP.NET web forms is so heavily used that it likely isn't going away anytime soon. Plus, there are so many custom controls and third-party support such as components and books. The main benefit of web forms is how easy it is to get a dynamic website up and going. It really is a RAD way of developing websites.
However, once that team has more experience with creating larger websites with much higher demands in terms of scalability, reliability, and test-ability, then they will look towards other solutions for that. In this case, they will realize that web forms are harder for unit-testing. They may also see that viewstates reduce performance and look for possible solutions.
Although MVC has the stated benefits, it is unlikely that anyone will convert their sites to use this new framework right away or ever. Plus, it requires the team to learn the new technology, and work out the new bugs. The team will have to learn new ways to do the exact same thing. For example, how easy is it to support uploading a file using MVC?
As I saw recently, there isn't a reason you can't create a site using MVC and web forms together. So you may see more hybrids in the near future. But I doubt that web forms will ever go away.
I kind of think about web forms like the way VB1 changed the way Windows applications are created on the desktop. To this day, the RAD way of creating application still exists and will never go away.