There's a lot of factors to consider here.
1. It was much easier to be 'genre-defining' back in those days. Here's a direct quote from one Ocarina of Time review.
For instance, when a bat is about to get you…that tune comes on "dumddududududu."
People were just excited that music was changing not only with location, but action as well. Now for people to praise dynamic music it has to be some insanely complex system depending on player actions etc.
2. "If one chooses a game from that period there's a high chance it will be genre-defining" is quite frankly somewhat of a misconception. You're not choosing a game from that period, you're choosing from one of the BEST games from that period that stayed in 'history' due to being so big and important. But if you actually go to a video game database and choose a random game from that period, the chance it was genre-defining is insanely small (don't get me wrong, doesn't mean it will be bad... though there's a very big chance it will be bad).
In other words, think less of Half-Life and StarCraft, think more of The Lord of the Rings: The Two Towers, Black, The Getaway, SpyHunter (all of which are good games mind you, I specifically chose these as some of example of best-sellers of the time that most people are unlikely going to call genre-defining or GOATs or even remember when discussing that era, and there's tons of titles like that which are not even best-sellers, and that is if we don't go into the huge amount of games of regrettably poor quality that were released in that era).
3. The time of constant innovation is over, the time of finetuning our craft is upon us, but... people don't really appreciate craft THAT much. Not yet at least. When I played Ghost of Tsushima several years ago I was in awe because it was one of the best open-world games I have ever played in my LIFE. And I played TONS of open-world games. It might not be Breath of the Wild or something, but you would be hard-pressed to find an open-world title with combat, stealth, world-design, connection between character/story/fantasy/progression, and a lot of other elements done as amazingly as in Ghost of Tsushima. The craft of this game is truly fantastic, so many smart decisions including very little details like the sound of enemy yelling after noticing you not propagating until a second or two in which means you can still keep your stealth if you manage to immediately kill the person who noticed you. *Chef's kiss*. Tons of other open-world games with stealth miss those kinds of details that when combined form a truly stellar experience.
And yet, since so many reviewers were lowering the score because the game wasn't innovative enough on a large scale, on Metacritic it has a rating of 83. Which puts it lower than a game like Assassin's Creed III where the combat system is very simplistic (but looks amazing not going to lie), stealth is literally broken (Jagers psychically detecting you from beyond the walls, not being able to hide in bushes if a guard notices you even for a milisecond, etc.) and tons of 'content' is just 'go here and interact with/kill this random dude'. (Not to diminish AC3's value in terms of pushing a lot of tech forward which was very impressive, plus there are other advantages/good things it has)
So.... TL:DR - yeah, definitely a myth.
