The internet has probably allowed for smaller niches to develop and survive. Years ago, no one would bother making new genre X music in their bedroom because no one would hear it. Now they do. So arguably your notability requirement for genres is a bit outdated and maybe is why it seems like there are no new genres or subcultures. It's possible the genres and subcultures are just smaller and less visible now.
Disclaimer, I know nothing about DnB. I'm just commenting on how it seems a lot of things have developed in recent years due to the pervasiveness of the internet.
I would never argue I'm not outdated :)... but I am sceptical of notability claims if they are not bringing people together at events or getting promoters trying to cash in. IMHO leokennis's examples of Amapiano and Afrotech which I could find as events in my city suggests it's a reasonable yardstick.
Disclaimer, I know nothing about DnB. I'm just commenting on how it seems a lot of things have developed in recent years due to the pervasiveness of the internet.