I don't want this thread to devolve into bashing another cms product and so far it has not, only facts have been mentioned that are straight from the source, The Upgrade Myth and the v5 RIP Announcement.
But I don't think this means there is any fundamental problem with MVC, its a good design pattern, but there is much more than MVC involved in building a CMS or web app framework. They are just saying that in their start from scratch approach they ended up over-engineering a monstrosity (I'm not being derogatory this is what they themselves have said). As I understand it, their plan now is to implement support for MVC and other improvements moving forward form their current code base rather than starting from scratch and pulling in some of the good ideas they had in working on v5.
It will be interesting so see how that turns out because there really is a fundamental incompatibility between WebForms and MVC. They can be used side by side within the same app but a given request can only effectively use one or the other so you can't mix and mingle much at all. You cannot use WebControls or UserControls in MVC, the life cycle of page events for example does not exist in MVC. So while data access and business layer logic could be re-used it stll requires a re-write of most or all of the web UI to change to MVC. I think it will be challenging to make that change in any kind of way that doesn't break custom funcitonality that others may have built on top of it. I guess if you beleive upgrading is not a concern then its ok, but I can't really go along with that. I've worked very hard to make upgrading mojoPortal a smooth process and we do have lots and lots of people who upgrade to every new release while I'm sure there are some who upgrade less frequently.
These are the reasons I decided not to rewrite mojoPortal in MVC. It would probably take a good 3-5 years to really do it for all features we ship (which I cannot afford to do on my own dime) and it would still break compatibility for all custom features that anyone previously built with WebForms. All that without actually adding anything that benefits end users.
MVC is a design pattern, that does help in separation of concerns and it appeals to uber engineer types for that reason, but my goal with mojoPortal is for the code to be easy to understand by average developers not to impress uber engineers. We get a lot of people using and extending mojoPortal that are sysadmin or designer types who can do some programming but are not hard core engineers, and that is our target audience. WebForms has its own advantages that are different than those of MVC and I think it is more approachable just as VB was more approachable than C, and the uber engineers who like MVC better tend to look down on WebForms in the same way that C developers used to look down on VB developers. But lots and lots of good apps were developed in VB because it was more approachable and more suited for rapid development, and there are still a large majority of ASP.NET developers using WebForms. Also some of the things that people don't like about WebForms can be solved by smart app design and also they continue to improve things with each version of .NET. So myself I respect MVC and can see it has its merits but I'm still a fan of WebForms and cannot see a business case for re-writing 7 years of good code written in WebForms in MVC.
Best,
Joe