How many of the people doing that are locals doing it as a regular thing, vs tourists doing it as a one off experience? The core argument in the article is that the younger generation aren't going to their local clubs regularly enough to keep them afloat, preferring going to do much fewer and more 'special' events. The places that can survive are those that either bring in lots of tourists and/or focus larger one off events that can pull in a really large crowd.
Maybe on a particular night, but on any longer timescale: how does one make friends who will tell you about the cool underground scene without first meeting them in the aboveground club scene? Maybe online stuff plays this purpose now but I assume its still mostly the former.
lots of places. the people at the warehouse rave usually did something else earlier in the night. maybe you met them at a bar or show and asked what they were doing later. “what are you doing this weekend” is a normal thing to ask anyone you meet in a third place. it’s not that big of a secret.