Ever since zombies became predictable to satisfy the "tower defense" aspect, it became stale.
It was fun the first few hordes just shooting like a madman.
But if I dont intentionally @%$# up a design or neglect building my hordebase, it is just a chore. A chore that gives great XP... but a chore nonetheless.
You can not have predictable behaviour in a horrorgame. Maybe a hand full of games have pulled it off... but as soon as you realize that their movement is predictable and you can work around it... it loses the fearfactor.
Yes A16 A.I wasn't great. No complex tunnels and running on the spot.
But at least you never knew what they would do.
You didnt build a hordebase (well you could but it wasnt necessary) because they would attack from all sides anyways.
So every night was exciting. And no that is not just me "knowing the game now" I have played more alphas before the change than after (8 before, 3 since)
But I think it fills the thrill for newer players, which is after all the only demographic that matters.