As you said, we've done this dance multiple times before. You and I in particular. But for the sake of argument, I throw out some bullet points:
1) Pokémon as a franchise is fundamentally built around portability. The physical act of gathering together and playing is something that Game Freak and the Pokémon Company value to a strong degree. The stereotypical vision of an MMO runs in direct contrast to this, with people staying home to gather in the game world.
2) The primary audience for Pokémon are terrible choices for a subscription service. A Free-to-play model isn't ideal either and seems to be something that Game Freak shy away from even if TPC are more willing to experiment. The current framework of the games doesn't really support the models either and would likely need to be reworked to accommodate in ways that could compromise the local multiplayer element.
3) Generation VI is already effectively and instanced online multiplayer game. With a bit of work, the Secret Bases of ORAS could even be expanded into player run Gyms. At that point, what else would you want to see that cannot be achieved under the current framework?
I could probably come up with more, but three points enough to say I sufficiently defended my statement.
All your points are valid for making the main series into an MMO but if it were made properly it would be another thing, IMO. The main series is portable. I would never want an MMO to replace the main series. I would want something supplement, a la FFXI/XIV. Your second point probably holds the most weight in my eyes, I've never really thought about that. But maybe they could pull something off like Guild Wars ? I think that was free, you just buy the games
As for likening what we have now to an MMO, its simply not true. If you don't understand why people would want an MMO that might be why. Its just not the same thing. The closest thing to MMO in Pokemon would be entralink and even thats not really that close. An MMO is a seamless open world where everyone can quest together and team up, battle, have a player economy, and a bunch of other things, and the game is built around that. Player run gyms could be just a small feature in a player owned house (secret base) in the MMO world. Its just a separate kind of experience, it wouldnt work with the way the game is now with 8 gyms and a 100 level curve. IF they decided to apply the same structure to an MMO somehow, it would have to stretch 8 gyms into a 80-100 hour experience and totally restructure it to be built around quests and experience, instead of just the one main story. Like lots of pyukumuku throwing. That was very MMOish haha. Of course, all of this could also be thrown out the window and they could not stick to MMO tropes at all and just do it totally different.
Anyways, as MMOish as not only this, but most games are becoming nowadays, its still not the same. Pokemon has some multiplayer features that are better and more fleshed out than MMOs bother to be for sure, but it would be totally different to play a Pokemon game from beginning to "end" designed as an MMO.
All of this being said, too, I would be satisfied and not need/want a Pokemon MMO if they really created a meaty co op experience, instanced or not. They should just add some kind of RNG bottomless dungeon thing where you can co op with friends and get lots of loot and experience together.
It would also be amazing if money had value and they added crafting somehow, like maybe you can specialize in different skills "Gathering" (find rare berries, stones) , "Crafting" (decorations, useful objects), "Cook" (rare candies, juices) "Fishing" and add some kind of auction house...lol but thats probably just me
I don't understand what people mean by Pokemon MMO anymore.
The online features of the main games and Pokémon Go are you interacting with tons of people.
What the heck does MMO mean then?
The lines do get blurry. This is basically the crux of understanding the concept. A multiplayer online game is not necessarily an MMO. An MMO is literally designed to be played simultaneously in one game world with massive amounts of people with you along the way. For example, Call of Duty has "quests", rewards, loot, experience, leveling, and is a multiplayer focused game and is played my massive amounts of people. But its not an MMOFPS. Monster Hunter has all the markings of an MMO, even partying up to quest and take down bosses. But its still not an MMO. A Monster Hunter MMO is one where all the players share a world/story/experience, and play together in a seamless cohesive world that persists even when no one is playing.