Previously (about 15 yrs ago) I was a manager at a number of Chicago Rock Cafes (a late night bar and restaurant chain) in the UK. It’s USP was that they only played music pre-1986 - the idea being that it was to attract 30+ customers as they had more disposable income and were less likely to get wasted and start fighting and taking drugs (plus EVERYONE knows the music was better then than ‘today’s noise’).
They were very successful - until it came to the stage that the people coming into their 30s, simply didn’t relate to the pre 1986 music. By the time that they changed their policy and started playing newer music they’d lost the opportunity to attract new customers and the brand was sold off and shut down as they weren’t making a profit.
I don’t think that Stern is quite in that situation yet, by incorporating more modern themes such as Marvel, Stranger Things, etc. but I would suggest that they need to start incorporating more themes which are now nostalgic to 30 year olds, rather than the 50 year olds they rely on so much now.