The problem starts if the game offers some different content mini games like pet battles or raiding. Then people who are only interested in pet battles, or raiding, start to complain that they cannot play their mini game without playing the real game.
I think the problem came with the concept of the “end-game content” in level-based MMOs, most visibly in WoW. This content is available for max level, properly geared players. This content is often competitive and group-based, opposed to the laid-back single player leveling.
This isn’t a problem on its own, the problem is that there are two completely different games under the guise of the same title. The goal was to give a reason for those who completed the leveling content to stay. The result was the rise of a class of endgame players who like the endgame and doesn’t like the leveling game. As a result, these players demanded that leveling becomes faster, especially of alts that they see not as variety of content but as tools for raiding.
The dev now has to cut into the original core game in order to cater to the players who like the distraction that was thrown on the game. This is a vicious cycle. The more you cater to endgame players (no matter what the endgame is), the more such players you get and the less original players stay, making the demands louder.
Blizzard made a mistake adding raids and hard dungeons to WoW. Instead, they should have published a separate title “Champions of Warcraft” which uses WoW mechanics, WoW graphics, but has no levels and players start with basic dungeon gear. They could also release a third title “Warriors of Warcraft” which features battlegrounds, arenas and other PvP content, with everyone equipped with the same gear, fighting for ratings. Those who don’t like the leveling but like raiding or PvP can jump on it instantly. Those who like leveling can do it forever. Please consider how much content WoW has. If someone turns 12 today and dad gives him the game he played when he was a single college guy, he could play all the Vanilla, BC, WotLK, Pandaria, Draenor and Legion content. This is great content, don’t even try to deny that you have fond memories of locations or quests! Except… he cannot because the mobs turn grey after 1/4 of the zone and everything can be oneshotted, making the experience boring. With World of Warcraft a separate title, it could be done on normal speed, making it sure that one can do all the quests + local dungeons with them remaining level-appropriate and the mobs need some skill to defeat and definitely need skill to defeat fast.
The “end-game” was a mistake. Leveling MMOs should have an end with a “freeze your subscription and come back without losing a cent when we have new content”. For those who like the end-game there should be sister-titles that are easy to develop (well, easier than developing everything into one title).