Just got back from another retro gaming convention. I've got to say, these things are on a steady decline. Maybe it's just me, but I've noticed less and less of a selection of games at these places, which is telling given than many of the vendors come from near and far to attend. The selection is just at a point where a lot of those big ticket, sought after titles, common or not, just aren't there much anymore. I kept overhearing people in search of those same high profile titles that I was hunting out more than 10 years ago. Folks still want to find that stuff today, and collect it.
Luckily I'm no longer looking for those sorts of games, as I have most of them. I guess that's the point, they've been put into collections. I'm mostly scoping out oddities and obscure hidden gems which fit into the category of not too rare, or valuable. I found a handful, not a lot.
Anyhow, I think the enthusiasm for these events is waning. A lot of collectors fighting against one another for what little is left in stock, I think it's indicative of the state of game collecting. Supply running low, while yet even more people become enticed every day to jump into the hobby. The level of collecting is outgrowing the supply from golden age of physical media, and yet they're desperate to stop producing physical media with today's games.
I've noticed this too, however I don't think it's because it's harder to find rarer/more sought after titles due to being locked up in collections. Sure, that is part of it to a degree, but I think it's mostly that retro video game collecting has about run its course.
Around the time I got into collecting, NES was all the rage, and most retro stores I visited barely had any NES games and when they did, it was either your ultra common stuff, or stuff they'd literally got in that same day, and would likely be gone in a day or two. This lasted until 2014 or so when SNES replaced it as the retro console to collect for, then Genesis, then N64, then PS1, and so one and so on. My point is that people who grew up with this stuff reach a certain age where they can go back and rebuy this stuff, however after a while, the amount of people who haven't already got into collecting for a certain console(s) dramatically drops off. I'd venture to say that 98% of the people who had any interest in NES collecting have already done so, meaning that pool of people who will enthusiastically want games like Ninja Gaiden or Batman is quickly being outnumbered by the amount of available copies. And each year I see more and more people downsizing their collections, or just flat out selling everything off for one reason or another. Eventually in another 10-years or so, nearly every person who wanted to collect for a console that came out in the 80s or 90s will have done so.
I knew this day would come as it has with more or less every collecting craze over the last 30-years. People that grew up with something reach their peak earning years, want to rebuy their lost childhood, prices go crazy, then these same people start losing interest while very few new people are jumping in due to that lack of connection to said collectible. It happened with Baseball cards, it happened with vintage comics, it happened with vintage toys, and now it's happening with retro video games. Genuinely rare games like Little Samson or Hagane will always be valuable and they will never be sub-$100 games ever again, and many of them might even continue to go up in price. However, anything that isn't at least an 8/10 in terms of rarity will either stagnate or drop in price from here on out.
So I think the lack of stuff or vendors at the convention you went to was less to do with a lack of inventory, and more to do with people leaving the hobby, both as sellers and collectors.
The reason this theory doesn't make sense to me, is that these days, all I see is veteran collectors getting out of the hobby and selling off their collections. You'd think that means more on the market and a decrease in value, but it's not.
I think sort of like you mentioned, generations do go through phases of trying to re-buy their childhood, but then instead of just bottoming out when that gets old, it just transfers into the hands of speculator types more and more. The folks who got into collecting back when I started and before, I think genuinely just wanted to own and experience the games. They collected for the love of old school gaming. These days, it's dominated by the resell types and the speculators. Retro gaming on YouTube use to be chalk full off people nerding out about their collections and their pickups. A lot of that kind of content is gone. Now most retro collecting oriented stuff is like Pheonex Resale "Oh my God bro, I just found this box of games worth $1000 profit", as well as sealed game grading, "rare" game worship and just like people going for "full sets" for the hell of it (I would say probably as an investment).
So I think that's where classic games are largely ending up. Gamers getting rid of their collections, and going into the hands of people stashing them away as future investment projects, with full sets, rare games, sealed games, just anything they think will be worth a fortune some day. So a lot of that stuff is not recirculating. Anyway, that's my cynical take.