Maybe it's because I'm younger, but I don't feel like Mortal Kombat and Street Fighter are as well known as Zelda, not even outside the gaming world.

Newsflash, a lot of people alive in the 90's are still alive today. What a bombshell right? 

So does this mean that beanie babies are still popular?

Probably you werent around in the arcade days

Please read his original challenge, he said "known" not popular. 

My point is that things that no longer are actively popular will get forgotten and things that may never have been as popular as them will actually be "better known."  Do you get what I'm trying to say?  

There's also another thing you failed to take into account.  It's the fact that new people are being born all the time.  Things like Zelda existed a long time ago, too.  However, with games like Sonic not as many people who are being born now will know the game.  So you combine the people in the past that knew Zelda with the people now that still know Zelda, it could very likely be more known in an active kind of way.

Gonna have to agree with MZuzek here. Zelda is one of the big pop culture franchises because it's been one of the main video game franchises for 30 years. It seems very weird that someone would know other video game franchises but have no idea what Zelda is. Unless we're just talking about like 20 year olds who have never played video games in their life, I guess I could see how people who were born when like Ocarina of Time came out and have never played video games might have heard of CoD and GTA but not Zelda. Zelda had a lot more pop culture notoriety in the 80s and 90s than the past two decades.

Anyway, Zelda is certainly not a niche title, it just isn't massively popular like some other series. Big adventure games only appeal to a certain base of players, and there are a lot more people who would rather pick up the top 3d platformer or the same shooting game every year or the same sports game every year than will pick up an epic adventure game no matter how great it is.

First, not all things are created equal. I Love Lucy was filmed in the 1950s, a huge percentage of the population was not alive then, yet the series is still a well known part of pop culture. On the contrary, something like a slinky toy was vastly popular during its prime, yet I bet a lot of millennials have no idea what it is. 

2nd, new people are born all the time? Really? Thought we lost the capability to reproduce decades ago

Even if we disregard Breath of the Wild, Ocarina of Time sold over 7 million and Twilight Princess over 8 million. These are not niche numbers.

Isn’t “niche” just a synonym of genre? So yeah, Zelda is an action adventure video game, that would be a niche.

The way I have always heard the word used is as a category that an author or writer specializes in, such as medieval fantasy. So IGN’s niche would be videogames.