The rate of technological advancement is growing exponentially, so the duration of an 'era' is becoming compressed.
Is it growing exponentially? Personally, I'd say that changes at the margin are becoming smaller. I think that the difference between 1940-1980 is much greater than 1980-2020.
As an aside, it's funny how static music has been for years. There are numerous bands that sound like they would have been reasonably successful in 1970 (and visa versa). 1920 vs 1970 is a whole 'nother issue.
I vehemently disagree with this, but agree this is veering off topic, so I'll leave it at that.