It goes in a slow upwards cycle
There will always be lazy people + people who don't want to accept change/want to make money more than anything, hence why stuff are recycled. But there will always be people looking for new ****, so it will end up mixing
There will be periods of stagnation of course where nothing changes, change doesn't happen at the same exact consistent rate
But typically gradually there will be change, it's just very subtle most of the time
Sometimes but rarely it's very fast, however. Not everything is black and white
Music will never enter a permanent stagnation though as long as the music industry continues