It has never been and will never be economical for a heavyweight to wrestle 5 rounds. That's why typically fighters who do wrestle literally 5 rounds are welterweights or even lightweights, maybe a middleweight could pull it off. Particularly for a 40+ year old man in DC, by the late 30s wrestling becomes hard in general for a lot of elder statesmen who say have hip or knee issues or even just regular wear and tear. Maybe there could be a generational exception. In general, I consider it elite level cardio for a middleweight to be just wrestling for 3 rounds. Particularly if it's grappler vs grappler, an example is Amosov vs. Storley, Amosov was so gassed after the fight he left the arena and didn't conduct any interviews, but against a striker in Lima he had energy for days left after the fight. So depending on what the exchanges look like, even 3 rounds can be difficult for even good grapplers with good cardio.
I think Adesanya vs. Vettori (I assume you mean 2, really either). Vettori is not a wrestler, really whatsoever. He did not train wrestling as a kid. He's western european, there's no good wrestling there, and his wrestling is I think a really clinical example of how people who don't wrestle well tend to wrestle. He's really a classic Dana White jack of all trades tough guy. He basically has no ability to transition from one takedown to another. He tries and tries and tries for one type of takedown, doesn't get it, tires himself then gives up. Lentz and Evloev are both grapplers so one shouldn't dominate the other. Vettori, if anything, is a good example of the old-school outdated wrestling tactics that are falling by the wayside.
Now you're right about the evolution of the sport in that there are techniques now to get up. More people implement things like butterfly hooks or are good at making top half float around and not be able to land effective gnp. But for every one of these evolutions there's a counter-evolution. For example, Lima was trying to use the cage to stand back up so Amosov used bodylock takedowns that threw him in the center of the cage. The placing the head under the chin and using intermittent strikes. Or if you're up against the fence, trapping the legs. Also, people are getting better at doing economical mat returns. I think a lot of the new innovation is incorporating judo elements into wrestling, making transitions smoother. Gone are the days, you're absolutely right, that you just double legged someone, lay on top of them and swung elbows. You have to know more techniques now. But that's true for any profession right? Software engineers today need to know way more than software engineers 25 years ago, but that doesn't mean software engineering is less relevant or more elitist. I don't think the threshold is nearly as high as you think to implement a ground game. It's certainly not restricted to some individual elite talents.
The best gym right now for wrestling/grappling in my opinion is unquestionably American Top Team Coconut Creek (Kyoji trains there for example). I think they have a great system down there. Sadly, much of the great grappling that I describe is in Bellator, due to Dana's preferences. But you can still see ATT in the UFC as well, even recently. Dana's intention isn't to eliminate wrestling from the UFC, it's just to keep it at a trickle. Recent examples, Movsar Evloev beating hyped Muay Thai prospect Hakeem Dawodu, ATT guy. Renato Moicano choking out Jai Hebert, ATT guy. Tony Gravely (a former D1 wrestler) who knocked out Anthony Birchak 2 months ago, another ATT guy. Sanford MMA is another great gym, known for striking but their grappling is good too. Many people saw Shavkat Rakhmonov last week, he trained at Sanford before submitting Prazeras. Phil Hawes, a juco wrestler, won his fight last month vs a bjj guy. One of my favorite guys, Derek Brunson, has wrecked 4 straight hype trains with his wrestling, including Holland in March, also a Sanford guy. None of those guys I just named are "elite" guys in the wrestling world, or even are prospective champs right now. They're not Gregor, or like Khabib, Usman, GSP, but the tech still works even though they're not elite.
So I think it's there if you look for it. And as you mentioned, it is true that wrestlers are also transitioning better because there's more inter-disciplinary work, and people mix takedowns with strikes and strikes with takedowns. It's both good an natural, I think Blachowicz's strategy against Adesanya, for example, to strike for 3 rounds to preserve his tank and then wrestle for 2 rounds, brilliant. And more people will implement plans like that. So I think wrestlers are actually only going to proliferate more as time goes on. The only thing that is passing to the wayside are the old methods because of the natural evolution of tech, but the fundamentality of the sport and its importance as a base hasn't changed.