See post regarding non-commercial GA being a minuscule number of Americans...
Saying the one and only great mass fly-in is an indication of "health" of what's clearly not a common activity, is realistically a bit of a reach. We have to be honest.
AOPA's regional fly-ins might be a better indicator of "health" compared to overall society, and they can't even find a venue to host one in the Rocky Mtn West or Great Plains, east of the Rockies, let alone get enough pilots to come to make it worth their time and effort. The coasts and Texas had them, and how well were those attended?
All we really have left is OSH. That's one of the reasons it does so well. People aren't coming out to the local field to see the barnstormers on weekends.
(Well, if such a thing we're even legal anymore and the airport in Podunk didn't have to have a 7' tall fence with barbed wire on top around it, because it has one RJ flight per day.)
I've pushed pretty hard on regional air show committees within a six hour flight of Denver at GA speeds, to offer fly-in camping or hard surface parking and discounted or reasonable room accommodations, in an effort to combine the two, and there's zero interest. They don't have the volunteer base something like OSH has and can't really pull it off.
EAA has something very special with OSH that can't be replicated at any scale.
Would I want OSH to have real competition? Not really. Then you'd have to decide which friends to go see each year.
But the closest thing to me that even compares is Copperstate, and it's no Oshkosh... And is probably a better indicator of "health" than Oshkosh.
There are multiple States surrounding Colorado that can't even put on a good airshow. If they can, it's a few performers and none of the big names. They can't afford them.
Even Colorado's two biggest shows can't usually book more than a couple of well known acts and Grand Junction only does their shows in years they can get a military team as a show anchor (usually the Blues).
Let's not even discuss what a fustercluck the Denver show turned into trying to accommodate the Birds this year. Logistical disaster is putting it mildly. They issued mass refunds to people who couldn't get from the highway to the venue.
If fly-ins and GA were commonplace and there were multiple local-ish events with the big names out here, it'd be a hard decision to drive or fly 1080 miles one-way to OSH.
It's never a hard decision. That says something. Mostly that we're in an incredibly small niche hobby.
Oshkosh is truly special. I wouldn't use it as a "health" indicator, though. It's kinda like judging the health of football by seeing if the Super Bowl is well-attended.