If street art is dead, it refers to a certain opinion or notion about what street art was and how it died, And yet, It's quite difficult to find a common understanding of what street art is or was.
We could think of Street Art in a broad sense as art in different genres made outside (in the streets) and free for the public to see. Then we would have to include art genres as graffiti in all its forms, murals, sculptures, performances, installations, music, dance, literature etc. One criteria could probably be that you could distinguish these works from business signs, commercials and event posters. This definition does not distinguish between public art placed or organized by officials or legally by private interests and art work which is done spontaneous or illegally in the streets. With this definition we could say that Street Art is still alive, even if time have made the boundaries between public and private interests, commercial and non-commercial art, and legal and illegal art more blurred and unclear. So, when Os Gemeos paints a huge wall legally in Boston on commission (which may or may not be paid), it's regarded as Street Art because it's in a public space and free and for everybody to see.
Another way to look at it, is to see Street Art as part of the Graffiti movement, a culture, which started in US, France and UK in the late 70's together with rap music, punk music and politically radical communities. It was illegal, anti-commercial, subversive, political and regarded as underground activity and vandalism by the officials and the establishment. In the 80's and 90's the culture grew, expanded and differentiated into a multivariate of styles, technics and inspirations. New York based train painters influenced kids in Europe, and French stencil makers gave ideas to US and Australian artists and so on. The graffiti and street art culture became international, but also divided into subcultures with different "rules" and views on what their work was about. During the 2000s, the globalization of graffiti and street art have continued and the respective subcultures too. When these subcultures clash, they are referred to as "Graffiti War.” I think Sever's work in Detroit must be seen as a comment on the parts of the street art culture that have been commercialized, and that the main representatives (the coffin carriers) are ruining, in his view, what street art is about, the spirit and values.
The situation is confusing because today, the same "vandal" can be exhibited in a famous art institution in L.A., and arrested for vandalism in another state or country; A stencil piece raising awareness about child labour made illegally on a wall in North-East London, can be removed and sold on a art auction in America for more than a million dollars; President Obama have a street art print in the White House presented to him by the conservative prime minister in UK made by an artist who have been arrested and fined several times for vandalism.
In this view, the street art culture may not be dead yet, but the idea of a street art movement is deconstructed and transferred into a pop culture with present "street-art-stars" which travel the world. The public follows and everybody with an artistic talent wants be like them. The visited city gets free publicity on internet worth millions. Organised street art festivals around the world even use the word "Line up" when they refer to which artists that will be presenting their works. When "the biggest of them all" Banksy is asked: "Why are you such a sell out?" His answer is "I wish I had a pound for every time someone asked me that."