Narrative podcast is a media format, not an industry. Conflating those two like some silly dimwit is very likely to lose you big amounts of money. QED.
"Serial" was a podcast that helped kicked off the craze. It was new, had the true-crime pull that fascinates women and men alike, produced with a fairly low budget, available for free download asynchronously (unlike radio shows), covered a current controversy (which it helped hype up, partly out of self interest in the attention economy) and had enough cliff-hangers to string people along and have people around the world awaiting the next episode.
There is one "problem" with this sort of format though: It takes time to consume, very much like a streaming series. And there is only so much time that shows like this can eat up. I jumped on the "Serial" bandwagon right after a close friend of mine got all hyped up about it. I listened to a few episodes but quickly noticed the cliff-hanger shtick on keeping people on edge wether the convict was guilty or not. IMHO the trick was somewhat transparent and it became less compelling after I noticed it.
I'm pretty sure the format is still out there and used by podcasts, but it never was an industry, since every regular person with a cheap-ass laptop and a free installation of reaper or ardour has everything they need to get going and producing their own narrative podcast.
Thinking that this is an "industry" in itself was quite silly from the beginning. Sort of like calling "cooking spagetti" an industry.