Well zombies are and will remain a great plot element for several reasons:
-in one even (viral outbreak) all the normal people are replaced with monsters.
Large crowds of friendly NPCs are hard to program and more often than not an annoyance in how they get in the way and how their needs must be accommodated, but if they're all crazy monsters over night then no problem.
-Zombie outbreak can happen so quickly it doesn't need explaining:
This is what distinguished mutants, which are also humans but only through generations of bad shit happening, in that time environments change, weapons change, vehicles change.
-Zombie enemies are easy to program and fun to fight
Wave after wave of melee attacking zombies are fun to mow down unlike if they had guns or lasers where you'd be forced to stay in cover picking away at small targets for how far away they are. Charging enemies are fun to shoot in how they get closer they are a bigger threat yet easier to hit. It's easy to program hoards of enemies to charge at you.
-Zombies are fun because they are human like
Inside us there is the inner psychopath constrained by our sociable consciousness, we have an instinct to kill people, yet at the same time we know people shouldn't be killed because they are people just like us. But people who are proportionally jsut like humans yet mentally and superficially transformed to be non-human, without any human qualities but just mindless animals... I think that is what is so fun about shooting zombies. All the fun of killing people yet none of the guilt of destroying fellow sentient life.
Problems with all the proposed replacements:
-Mutants: how do you explain them appearing and yet no friendly NPCs around?
-insects: not human enough
-people: Like a lynch mob? Too smart, they would all run away after you killed 20% of them. Part of Zombies' charm is they are relentless
-aliens: too likely to depend on a few number with projectile weapons
-cyborgs: again, too likely to be few in numbers with ranged weapons
-Robot: Same problem as cyborgs
-Gangsters: They are a minority criminal element, smart and sane interested in self preservation
But what about Bioshock's "Splicer" antagonists. They are intelligent yet clearly insane and seem to be the only surviving inhabitant of an entire city, I'd think they'd be the closest stand in though a lot of exposition was needed to set them up as to how the socio-economic structure combined with their addiction to genetic modification. Then again, they also mostly depended on a few numbers with ranged weapons which isn't quite a suitable replacment for hoards of bodies attacking with mindlessly and relentlessly with close ranged attacks.