I mean i don't feel like this is even subjective. It's online gaming.

IMO I feel like online gaming shouldn't be referenced. Online gaming existed in the 90s and even the 80s. Sure, it's vastly more popular, but so is gaming in general.

Now if there is something specific within online gaming that you feel pushed gaming forward that occurred in the 21st century, that makes sense. But it would be tantamount to saying analog sticks in the 21st century pushed gaming forward when N64 and Dreamcast included those by default even though the way we use analog sticks today in games has changed and become more universal and analog movement is far more popular now than back in the late 90s.

It didn't really exist to 99% of gamers in the 80s and 90s unless you were rich, it was just far too slow to get a good experiance and for the majority of the world it wasn't even possible. Like being able to play street fighter, mario kart, MK, halo, rpg's and sports games etc, was a dream that was finally realized  in the 21st century for 99.9% of  gamers.

In 1999 average connection speed was 56 kbps and only 150 million people in world had access to internet and half of them were in USA

For all intends and purposes there was games with online capabilities but "online play" as a synonymous for MMOs did not existed 

I think that was a natural progression of the internet. I mean, people in general had less access to the internet. Of course as the internet became more popular online gaming did too. 

I just don't think you can give credit to the 21st century when it was already happening in the 20th century, and the trend of increasingly wide internet adoption was already happening too.

Ultima Online came out in 1997. Neverwinter Nights came out in 1991. Everquest came out in 1999. There were definitely MMO's. Even Counterstrike released in 1999. Not an MMO, but still something that is synonymous with online play. Starcraft came out in, what, 1998? That game was HUGE for online gaming, and is STILL active to this day.  

I just don't see why many are suggesting that this is a 21st century thing. It really isn't. It's like suggesting that 3D gaming is a 21st century thing, because 3D games are much more popular now than back then too, and they didn't exist quite to the extent they do now. 

Those games were, but virtue of infrastructure, unplayable for the majority of population. I personally don't feel we can call them MMO when they lack the meaning of the fist letter (Massive). They were online playing at best 

I'm aware first generation MMOs exist, but they were in no sense relevant. In the same way motion controls and VR technology already existed way back, and even 3D existed in the 80s. But when talking about industry availability and historical context are equally important. 2nd generation MMOs are the ones that ultimately changed how games are designed and people's relationship with gaming, and 2nd generation MMOs is a 2000s thing 

Online play and stuff like dlc/microtransactions/patches.

All for the worst thou

Okay, then talk about what FEATURES in those games or experiences that are isolated to the 21st century increased that. Saying "online gaming" doesn't do that. Online gaming existed back then. So what feature from the 21st century was implemented that increased the popularity of online gaming? 

Okay, availability and historical context are important. Well games in general are far more available than before. Back in the 90s there were probably about as many people who played videogames as those who had internet. Maybe a bit more. The NES cost around $700 when adjusted for inflation, the PS1 about $620 games were over $100 too. Computers couldn't just play any game they wanted to...and smartphones weren't massively popular until the iPhone in 2008. Now we have 3 billion people who played games, with many of them playing anywhere they want to. Videogames have made a greater cultural impact in the 21st century, with Fortnite hosting concerts even. Would it actually make sense to suggest the 21st century is responsible for "videogames" though? Or would we list out what features or things made videogames so much more popular in the 21st century?

Yeah this would be my pick as well.

The first time I swung the Wiimote to swing a sword in game, or aimed with gyro controls, felt every bit as amazing as the first time I played a game in 3D; that same sense of a whole new world of possibilities opening up.

Even now I have a PS5 I still choose to play shooters and such on Switch if I can, just cos motion controlled aiming feels so much better to me.