There are many threads arguing what an MMO is or is not.
I'll just lay it on straight. The reason no one can agree is because the definition is outdated. Simple as that. Some people cling to the old way, but most have moved on and embraced the new definition.
Lets look at older MMOs
Asheron's Call. Ultima Online. EVE Online. Star Wars Galaxies (as just some examples). All massive worlds with no instances or small co-op dungeons. They could be played by thousands in one single world.
Todays MMOs? Far more complicated and fluid. Today, many MMOs use co-op features instead of MMO features. Many themepark MMOs heavily entice people (with far better items than crafted) with small man parties, like WoW its 6 player limit dungeons. That definitely wouldn't be considered massively multiplayer in the old days. Does that make WoW not an MMO? No, but it is a great example of how MMOs are no longer the same from before and how the old definition doesn't apply.
Then you have MMOs that aren't very massive anymore, just the potential to be massive. Look at Istaria and there 5-10 off-peak active player base and then only 40-50 at peak...it has the potential to be massive...sadly it isn't and its a dead MMO. Still an MMO, but not really massive as it once was and in fact is smaller than most minecraft or even call of duty servers.
Does it make Istaria not an MMO? No, but that is where the fluidity of the definition comes in. People still consider it an MMO, because the definition of MMO itself has changed. It doesn't mean massive amounts of players or a huge living breathing world. MMOs are games like GW2, co-op experiences like WoW, or games like Destiny 2 that have many MMO features and feels more like an MMO than many MMOs do.
Does that make the other games not MMOs? No, because its a fluid definition. Many games could easily be considered MMOs in todays world. MMO definitions from even in the early 2000s no longer apply to today. The MMO genre has changed greatly, and now only oldschool MMO gamers are left not embracing the change. But they too will eventually give in and be with the modern times.
MMOs have changed, and that too means what is considered an MMO or not has changed as well. MMOs have evolved to be many types of games, and the old definition is outdated and no longer applies to newer MMOs.
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