Wall of text head. So either scroll really fast or grab a comfy seat.
There is also the fact that, at least with WoW, the story becomes a joke depending on how long the game runs for. I personally grew very tired of the constant conflict (If you can call the constant bickering and slapping between the Alliance and Horde a conflict) that kept cropping up between the two factions. Which results in quests where you mess with NPCs of the opposing faction. After a while, it becomes little more than a Saturday morning cartoon or an online comic strip with how things play out.
The game would also be grindy, and most likely a lot slower paced than previous Diablo games as there will be a monthly subscription that will entice the developers to create such content. Whereas now in Diablo 3 you can get a set of equipment in a week or two, expect it to take months to get that same equipment, let alone level up.
With the route World of Warcraft has been going over the last few years, I definitely would not want Diablo following the same path: super grindy reputation quests, locked races, boring quests, onion layers of equipment and RNG; faction leaders going all derp before going bad, and the changes to classes every expansion pack.
To put the changes into perspective, it would be about like everyone being forced to use a set for 1-2 years. Not having a particular set rewarded every season, not to be able to make your own tweaks to the set that may or may not be top tier, only a single set. Also maybe 1-3 legendary items along with it, if that. Then next expansion pack they may or may not swap sets around, forcing players to change how they play, or they make keep things the same but slam a class with the nerf bat so hard theyāll suck for 1-2 years.
That is part of what drove me off from WoW: I used to play the Death Knight class, specifically Unholy, and had from the beginning of Cataclysm up to the beginning of Legion when they changed the specialization, as well as overall class, so much that I hated it. In Cataclysm, I was so in love with the specialization I had an AoE variation and a single target variation. When I was given time to swap back and forth I was able to keep up with death knights who had better gear then me, barely. However with every expansion pack the class, as well as specialization, was dumbed down. They kept taking customization away, ability to really tinker with setups, until it all became a shadow of what Death Knights were.
I went warrior, which I liked, but I was eventually driven off by how legendary drop rates were, ironically, Diablo-like. While in Diablo 3 it is the norm, in a game where your raid group can only tackle content once a week at certain times, requires organization, as well as really good gear to tackle the content, locking legendary items behind a random drop system really sucked. That is not including the different variations of armor on top of the different tiers that raids have (normal, heroic, and mythic): Titan forged and whatnot. Last I heard, in Battle for Azeroth there is a means of upgrading the armor, but one that supposed results in the stats or buffs being reset. Do not quote me on that.
Long story short: I do not want Diablo to become another MMO as, if not immediately, will gradually sap away at Diablo. Of course with Diablo: Immortal on the horizon that is happening regardless.
