Don't know which market in particular? My background is in the club industry, specifically house music like mix mentions, prior to focusing strictly on the private event market. My experience may be from a different generation, but similarities of each generations trends seem to follow. And while I have said I worked with one dj... I know many "house" dj's personally who aimed to do maximize both private and nightlife events. This gave me exposure first hand to seeing what the difference was between those who adapted and survived compared to those that tried to hang on as long as they could to what they loved and were known for during their peak, and just seeing them slowly fading (or at least scaling back big time to how much they work or the amount they work for).
To be fair, I'm focusing on the private event aspect (you are totally right about public events and nightlife, which I'll mention). The private event industry is the industry where being known for ONLY a certain style of music (or event) can potentially be a negative. For example, the house dj's in the 2000's up till 2013 were all in demand, and many were doing guest appearances at private events, or running a private event company and being the company that people wanted to hire. But with the majority of that generation now entering their late 40's and 50's, the private events that they throw are becoming less and less, and therefore the niche that these dj's specialized in is in less demand. If they didn't rebrand as djs they would be disregarded as a dj that "only" knows a certain type of music... one that is no longer in style. Many of course adapted and now play open format or are more wedding friendly or whatever was needed to keep themselves busy.
You are correct in saying that this particular demographic that you reference is absolutely still relevant to the nightlife and public event industry. There are MANY events consistently going on, and there is still some work to be had for dj's that want to still be known for this music. Mix would in fact be in a much better position to keep himself busy if he decided to spread his wings a little and look at different parts of the state for work for the music he likes. Areas such as newark, elizabeth, linden, perth amboy, etc have many small spots that welcome the music he likes. But again to be clear... I am focusing on the private events.
Personally, when I first started, I tried to brand myself as the "greek american" dj... and while it helped me grow within that community, I could not break out of it. I'd often get calls from prospective clients who were given my name and the first thing I'd get asked was do I play american music, because the site and business cards said specializing in greek american music. After I dropped that tag line, I stopped receiving such questions. Now my tag line is Luxury Wedding and Sweet 16 DJ... and that works for me as I want to weed out other events... one day it may not include sweet 16... as I get older and have to scale back I may have to take the Luxury part out since I may be too old to attract the modern bride, but will still need to keep busy.
So my overall point is in saying that Mix's focus on such a specific type of music, and being explicitly known for that, alienates him from the majority of Private events that take place around him.