I don't normally comment in this section as I have absolutely zero involvement with what would traditionally be described as "vintage," but feel as though perhaps some might be interested in how the younger generation views this manner.
I'm a mid-20s guy who recently left his job and went back to grad school. For what it's worth, I was raised primarily by my late-boomer father and greatest-generation grandmother, so my worldview is not one of the typical "Millennial." Since high school, I've owned a smattering of GRM approved cars (79 Corvette, Foxbody, C5Z, Neon ACR, and a few lesser Neons, the first three I still own). From my perspective, there's a few issues at play here. I will try to address them in no particular order.
1) College and post-college employment. I was fortunate. I dropped out of Engineering school at 20 and knocked out classes at Community College to keep my mind sharp. Spent time working behind a parts counter. Landed a "real" job in the medical field where practical knowledge > a degree. Made decent money, bought whatever I wanted, vacationed, etc. Always had 2-3 project/fun cars kicking around. It wasn't until I decided to "take the next step" due to boredom with my (then current) job that I decided to head back for three years to finish my Bachelor's and obtain my Master's
Unfortunately, many are not so fortunate. They picked poor degree programs, etc. This is pretty well documented and I won't get into it. I will not say that the younger generation are not into cars or racing, but it is perceived as a hobby for more affluent people. The reality is that combining the lower incomes of the current generation and their poor spending habits have a lot to do with this. While I never received negativity from people for owning 4-5 cars at a time, many asked how I afforded it. The truth is, it's easy to afford a car payment, a restoration, etc. when you cut out going to the bar every week. That alone is worth a few hundred dollars per month.
2) Current automotive enthusiast landscape. I blame the internet A LOT for this. Things like GRM do not exist on the rest of the internet. On a daily basis we are bombarded by the next "big build," with cost-no-object engines, motorsport-grade electronics, etc. It's no longer enough to put together a clean driver with mild engine mods, decent suspension, etc. If you're not running high-compression, forced induction, Motec ECU, Racepak, or similar, nobody seems to be excited anymore. I won't touch on visual enhancements because they really don't have anything to do with this topic, but that certainly plays a part. People are so jaded by SEMA/sponsored builds and other craziness on Instagram that they're out of touch with reality. Even though I offered to completely fund building one of my Neons into a Chumpcar, I found few friends who would personally be willing to put in the work. Either they were not mechanically-inclined or cared little about road racing because it wasn't "cool."
I've noticed that a lot of the interest comes in three flavors: hardparking, HPDE, or drag racing. Scroll Facebook videos and you'll see a ton of content from places like 1320 Video showing 1000+ HP cars doing highway rolls. I love how road racing has maintained some semblance of tradition because I appreciate those sort of things, but they do a poor job of reaching new demographics. This is one area where events like Gridlife have really capitalized.
When it comes down to it, the era of taping your headlights and heading to the track are over. As mentioned previously in this thread, I feel ego has a lot to do with it. For whatever reason, the current generation is so risk adverse that they would rather stay at home than be "humiliated" while competing in any sort of activity. Likewise, the ability to afford and house 3+ vehicles (tow rig, DD, race car) and a trailer is not something many can muster, let alone care to deal with. Most would rather just dump money into a better street car.
3) "Vintage" car classification. This has been discussed before. I feel the suggestion of letting in old IT cars would be a GREAT idea. There are tons of "outdated" GTIs, Omnis, Escorts, Neons, Hondas, etc. that have no place to really play. Craigslist is littered with old IT cars for a few thousand dollars. While they might be heavily outgunned at a normal SCCA event or similar, these would fit right in with a vintage event. Not too many GenX/Millennials care about Sprites, TDs, and the like as they have no real emotional connection to those cars. 80s/90s stuff is pretty hot right now, and only growing in popularity. This eliminates a lot of the more affluent/established younger people who either end up doing HPDEs, go back to street cars, or MAYBE entering something like Lemons or Chump. Moreover, current generation car guys are A LOT more comfortable with ECU tuning than they are synchronizing SU carburetors and using a timing light. Failure to adapt (for lack of better verbiage) definitely has a lot to do with it.
So this basically boils down to a cost issue, but not necessarily because "racing is too expensive." I really tried to make this into something other than a "Millennials are killing X" rant, hopefully things did not come across too negatively.