Pop music will evolve over time, but I do think it will happen slowly and that the changes from the current format won't be large. At least not until something very dramatic happens to society.. talking western society here in broad terms.
To repeat something I recently wrote in a different thread, a song is made out of various musical traits. It could be distorted guitar, a 5/8 time signature, the break or whatever you call it in Skrillex-songs and so on.
Many of these traits are competing against eachother. What they're competing for is our love and attention. Let's say time signatures.. Music in 4/4 competes with music in 13/8 and other time signatures, possibly also a mix of time signatures within a song. What happens is if the audience finds music in 4/4 more appealing than music in 13/8, then music in 13/8 won't be commercially viable and so will become more obscure. This means that generations growing up will mostly listen to music in 4/4 and will be shaped by that, causing somewhat of a reinforcing feedback loop. While music in 13/8 will still go on existing, it won't have anywhere near as high an influence on society as songs in 4/4. These frequencies/ratios between traits can probably become quite stable for long periods (talking decades or more).
And so it goes for various traits in various genres, whether we're talking harmonies, timbral palettes, instrumentation and so on.
Going up a level, songs compete against other songs. From this perspective, it's about having the right combination of traits for what kind of song it is. The "right" traits make for more competitive (successful) songs. Over time, pop gets distilled into a kind of music that features the most competitive ("right") traits.
What you might ask then is are the "right" traits going to change? Broadly speaking, there are two things that determine what the "right" traits are:
- Nature - we find some things naturally appealing and other things not. I don't think baby crying is ever going to become a very prominent sound in pop music.
- Culture - you know what this is
If culture changes - and so the environment that those traits exist in changes - then that might in turn change what we (on average) consider "right" musical traits. I do think that the traits that we have now are quite distilled to what's "right" for our current nature/culture, but perhaps there's some unfound innovation coming or perhaps society in a hundred years will be very theocratic and lay down some hard rules for what sort of music is allowed or not. Who knows?