Your example leaves out a ton of aspects and how the MMO landscape has changed since then. First of all MMOs are not strictly comparable to regular single and multiplayer games.
Back then MMOs were very grindy, that much is correct. However you have to take a moment to think about the reasons for that. One of the main reasons is that in old school MMOs socializing was a very major part of the appeal and made up large portions of your gametime, you were required to cooperate and coordinate with people manually (form groups, guilds, trading etc.).
WoW is a good example, because it was very similar back in 2004, but made many aspects, especially playing alone much easier than e.g. EQ.
So what does it mean when people say that modern day WoW feels more like work than oldschool WoW? The reason is that many many social components are gone from modern day WoW, it now has automatic systems in place for pretty much every aspect of the game, most content is soloable, and the parts where a group is required offer systems to automatically search and find players etc.. In short: basically no manual communication is required in WoW nowadays to see and complete almost basically everything the game has to offer. There is one exception: there are versions of parts of the content that is just harder versions.
What makes modern WoW feel more like work than oldschool MMOs? The reason is that the game design intentionally keeps you locked in a very tight loop, many parts are designed in a way to keep them from being completed too quickly, many parts build on "daily quests" that can be completed exactly once per day/week, but no more than that. There is much more nuance to this, but in short it can easily create the feeling that you have to show up to play certain content you don't enjoy and if you don't you lose progress/time you can't catch up on. Think "oh no I still have to login today to get my daily quests done...", players typically complete missions/quests like that not because they are fun, but because they are working towards a reward.
But since ingame mechanics have killed the need for social cooperations and interactions (this is not strictly a negative thing, and a hard problem to balance) it creates an atmosphere that is a very different flavour of grind than back in the day, which is closer to some kind of work obligation as opposed to a virtual world you go to to adventure and meet with other people.