If one person knows about it and that one person fishes it and that one person keeps their mouth shut, nobody else knows and it will stay under the radar.
FWIW, there are no streams in PA that are on the class A list that I would consider to be "under the radar" today. Some are more popular than others, sure. But none of them are unknowns. The fish commission has "outed" them all. Every last one of them that's on public land gets fished reasonably often. And if those experiences are good, they're gonna get talked about and become reasonably popular. It's just the way it is, and there's nothing you or I can do about it.
Are there other large streams which produce large wild fish? Yes. But they are either 1. not class A or 2. Not under the radar.
And Penns has plenty going for it that makes it get extra attention. It's big and long, which is a rarity on the class A list. It has a high alkalinity, which shows up on the class A list for those looking for limestoners. It has 2 special reg zones, and that list is a big neon sign for visiting fishermen saying "fish here". It gets stocked in multiple places, so even the truck chasers see a large stream which gets stocked over a long distance. It's got tunnels and hiking/biking trails, campgrounds and state parks. i.e. it's popular for more than just fishermen!
Now, if it were not constantly discussed in social media, as well as newspapers, magazines, books, etc. as being among the best in the state. If it doesn't have a well known fly shop on it's banks. Etc. Would it be less crowded? Sure! You might lessen that "destination" feel a bit. It doesn't get on bucket lists, and people don't come from all over the country just to say they've done it like some sort of right of passage. Especially at peak hatch times, it might not have so much of a circus feel. And I'm fully in agreement that this aspect of ANY stream is a little ridiculous. But it would still be a popular fishery. And honestly, outside of maybe late May, I don't think you'd notice a big difference in the number of anglers. They are mostly regional, and they are going to know it for what it is. The "destination" chasers are almost all coming in May.