Because this question was asked to you in a job interview, I'm not sure we should answer it here, since then we kill a great opportunity to separate the knowledgable from the less knowlegable :D
But anyways, here goes (I've basically presented the answer multiple times when talking about DXA):
In January 2008, Tridion R5.3 was released and the DWT Templating model introduced. This offered the ability to publish ASPX pages, versus ASP/JSP only from VBScript Templates (not entirely true ;o).
However, going forward we (the implementers community) found that DWT Templating was not the best option for dynamic websites, since you were doing both CM side rendering and CD side rendering (at request time of the Page). Also publishing ASPX pages separately has a performance impact, since they will need to be compiled, slowing down your application server with each publish action. This all is by the way just as valid for Razor Templating.
Now the evolution of the internet has demanded over the last few years that we build more and more dynamic website, with media replacing textual content, campaings owning the homepage and content needed to be personalized and optimized for visitors as well as devices.
So this means that we needed to look at a proper application model for the web site implementations. MVC is one of the solutions here, and that requires a different Templating approach in Tridion. The first attempt at implementing MVC with Tridion was called CWA and that was only available for Java. DD4T is basically its successor and became available for both Java and .NET.
The next benefit of using DD4T (or DXA) is that you can leveredge MVC web application knowledge for your implementation, since these frameworks solve the Content Provision for you and handle the connection to Tridion, so the web application developers don't need a lot of Tridion knowledge to do their thing.
To answer your specific questions:
I already mentioned the development and knowldge benefit of MVC, and it also is a lot easier to debug and troubleshoot. Since you can simply run the web application directly from Visual Studio, where as with DWT Templating, you can't.
Hard one to answer, since it depends on what you want to upgrade to, there will be different benefits in each when upgrading to a newer version of tridion vs a newer version of the ASP.NET/MVC framework.
A database is much easier to scale than a filesystem I would say, but some might disagree. But when talking about the web application itself, an MVC web application to me is more flexible and scalable than ASP.NET WebForms