If we get rid of the system of fixed monster spawn points, and just have monsters appear in spots where there AREN’T any players, then we will take away the ability to “farm” a single monster.

Not necessarily, so long as the MMO in question retains what I feel like is the major factor in encouraging farming — That monsters pretty much just stand around (or patrol) and wait until you get near them. Having monsters that react to players the moment they see them, rather than when they are within an artificial “aggro bubble” would mean players have far less control over the encounters they initiate.

Guild Wars (just to give an example I know a bit about) has pseudorandom spawn points for all enemies. Someone who has a build specifically to farm, say, Trolls in Guild Wars is going to seek out engagements with Trolls and run around any monsters he doesn’t want to deal with so he doesn’t waste time. It’s not always possible to avoid engagements but it’s far easier to avoid them than in say, Diablo where enemies will largely attack on sight and might begin attacking you from off-screen.

Really, the fixed monster spawn points is an awful system and is long overdue for innovation.