I've written about this at length before- but the first time vinyl crashed was a mass exodus to a superior sounding new format (CD) and the reason the comic/trading card bubble burst was that the majority of the target audience grew out of it leaving wannabe speculators holding the bag with tons of product. I don't need back issues of 90's X-Men anymore- i do and will always need my Hendrix Family reissues.
Predicting a vinyl bubble burst is to either predict music fans are actually going to abandon physical product for good, or that there is going to be something else that will supersede it as far as quality and collectibility. My opinion is the reason the "bubble" is still going over 15 years later is trifold:
- There isn't as much competition for media collector's dollar. With streaming TV and movies, and with 4k not really the game changer that BD was, a lot of people aren't splitting their entertainment budget with TV show box sets or movie upgrades anymore.
- You don't have to buy everything that you kind of like anymore. With Spotify around to scratch the gratification/preview itch, consumers can pick and choose what albums they want to have a special physical copy of. And nothing is taking it's place; we've been through it all at this point and vinyl won out.
- Third and most importantly: Ten years ago the record labels listened to us. A ton of those LP's from 2007-2010 sound like trash. Listeners wanted quality for the price premium. They responded by contracting guys like Bellman/Gray/etc on overdrive to make definitive sounding reissues of tons of albums. So the quality is there.
So I think what you actually have instead of a bubble is a millennial generation (aprox with some X'ers and Zoomers on both sides) that have survived the Recession of 2008 and now the COVID crash and are using their income to fuel a record industry- maybe as a form of escapism (I don't know enough to expound on that). If SHF is any indicator of how long that type of buying behavior can last, this should be going for a long time yet.
You say it has won out as the physical format but CDs still shift more than double the units that vinyl does. Vinyl has now surpassed CDs in terms of revenue generated, due to its much higher list price, but is still a fair way off in units.