In terms of (just to pick one) blues, you also have to remember that blues is at its heart quite basic. It was the way an awful lot of great guitarists learned to play, and initially they learned because it was easy. I can't play but it's relatively well known that one of the easiest scales/progressions to play in music is the twelve-bar blues, in fact it forms the basis of an awful lot of music. If these guys (Muddy, Sonny Boy, Wolf etc) had all been trying to learn classical or jazz, chances are it would have been a lot harder (though I'm sure some of them later moved into jazz at least, and some may have been playing it, but not at the start), but blues is simple, uncomplicated and with a few chords you can make a song, so that would have been attractive to young guitarists at the time. Not to mention that, especially in Delta Blues, an acoustic guitar was all the guys had.
But blues is still being played, and improved on today. Look at Robert Cray, Eric Clapton, the late Gary Moore and Rory Gallagher ... all played blues to the highest level possible and incorporated it into their music. Sure, blues could be seen as being stagnant, but there's not the widest amount of room there for changing it. It has formed the mainstay of rock music for over seventy years now, so it's hardly fair to compare it to post-rock and say it's a stagnant genre.
Love your avatar, by the way: reminds me of the fox off Glacier Mints!
