Mercs worked initially for what they were supposed to do: help new characters level faster and help bad solo classes get by. They were NOT meant to be a substitute for an actual group; the whole purpose of EQ (at least still at that point in time) was to have at least a bit of a communal experience and not play a single player game with other people in your space. They were the same line of thinking that saw stuff like TBS' aug quest - let people do something while solo, but grouping is still better. I do not think multiboxing was an intended direction for them at the time.
This is naturally why the remaining success of EQ tends to be in live guilds where there is raiding and on TLP where there is true box. Having multibox armies seems cool and certainly sells those subs, but, it runs into two outcomes most of the time: either the plat farming (often fully automated) who are a degenerate presence in your game, or the Tuco style play for new content and then put it back on the shelf for a while type - good, but not a very sustainable model for a game in sunset mode. Without community in your MMO, you may as well just go load up a non-MMO with online capabilities.
So, tldr, a good MMO needs to have social and community factors be superior to solo play, but have SOMETHING solo that can be productive. You want people to play with other humans but you do not want them doing absolutely nothing in the meantime.