I don't think rock n roll killed the accordion, it actually made it that much more cool - in the eclectic sense of things. I mean, how many guitar players are there nowadays besides everyone!
I can't help but wonder if rock n roll has indeed reached the age of retirement in terms of how old the genre actually is. Most things have been done, and while there are still bands out there I discover from time to time that are really great - you can only do the phenomenon of Led Zeppelin or the Beatles once and nothing will ever be as good as those things were unheard of. I think a lot of the popularity had to do with the young rebelling against the authority and for the most part was a simple form of genre that branched into a number of complexities. So it had to be new and exciting, and definitely loud. It has now gotten to the point that for me, it is just loud and obnoxious and lost its way. Having said that I think most consumers of music are just that, and it fits well for the working yet not so thinking class looking to get their angst out on the weekends and after work.
I think accordions and other folk instruments are on the rise in terms of popularity again but probably not the extent it might have been; partly because they have tones proven to the pass the test of time, partly because they do not consume electricity, partly because they use less gas to haul around and definitely use less space than all of the trappings of electric instruments. Also because connections to the physical world are being lost due to digitalization and there seems to be an increased sense of longing (oh the urbanity) for that physical connection for a lot of people feeling trapped by that, which acoustic instruments really do quite well naturally. They also seem to get along better with your neighbours.
I spent more than two decades searching for electric guitar tones, and while I did pretty well with it I was surprised to learn the natural buzz and distortion of the accordion to definitely be superior and unique to all of that.
I do think of the accordion as the Da'vinci of instruments, and as such is too smart to kill. However in many regards, it might actually be too smart for its own good to be available for the average citizen who just wants to rage after a long, hard and bad day, and that's ok too.