Of course, you can no longer clumsily bring a pure mobgrind today. At least not obviously, because it still has to serve as a basis, of course. It is just hidden much more skillfully and with a better adventure feeling and rewarding and motivating game experience and no longer seems like a stupid grind.
In addition, the development is also constantly coupled to it and the world, as it is typical in an RPG, is only given a kind of reward in the form of a quest, including a good story behind it, when you explore and reach certain passages.

In addition, so many things are feasible today that could be implemented much worse back then.

There are a massive amount of players who would love this kind of MMORPG, and even more players who have been waiting for this kind of MMORPG for ages and don't like the current endgame and solo games at all.

It's high time to bring back a real MMORPG. Proper in the sense of how it actually means the genre.
As I said, today's modern MMORPGs are not really MMORPGs according to the genre.
Even the many aRPGs tend extremely in this end-game tunnel around bosses and items.... D3 has also ruined itself, among other things, and D4 will meet the same fate, if you think to forget the actual RPG game and build an endgame game.
D1 and D2 were successful for a reason... They were still real games, without an endgame and that's why they were so good.

Endgame eats up RPG and its content and solo quest games massively prevent all the typical elements that make MMORPGs MMORPGs in the first place...