Using cshtml for views will require more from a "HTML" developer.
Using Bootstrap (or a similar framework) should reduce the need for highly specialized CSS developers.
But I would say that this increases, not reduces, the need to be able to differentiate between backend and frontend. You need to be able to say "Frontend developers need not apply" (in a polite way, of course).
My guess is that in the future in the ASP.NET world the Frontend role in a team disappears but the Backend role will be even more visible in job descriptions, ads.
That is not that different than EntityFramework is reducing the need for specialist database developers (though not for the need of knowing how to handle database performance and how to design databases, it is now a part of everybody's role).
The focus will be on getting developers who can understand and implement business logic. And that is exactly where the focus should be. It is where value is added for the company.