Alternative really isn't a musical genre. It's a radio format. It's a label -- and a flexible one at that. It's also a convenient term used by some music fans to describe their favorite music so they can appear stylish. And it fools lots of people.
I remember when the top three or four selling artists in the U.S. (during the mid 1990's) -- we're talking millions of albums, millions of fans, albums entering the Billboard charts at number one -- these artists were still considered "Alternative" by the radio industry. "Alternative" acts -- after three or four number one albums. "Alternative" to what?
I remember asking one of the radio programmers I knew at that time "how can these acts be considered 'alternative' when they are the biggest acts in the country, and their style of music is actually more mainstream than "mainstream rock"?
The answer I got was, well, 'just because'.
It's just a label. A convenient label.