So just what was Belial's plan there?

[NuurAbSaal]

It's obvious that they wanted to make the main story simple and linear so that everyone coming to the game could follow it and know exactly what to do at each stage. It wouldn't surprise me to find out that the writers wrote a more complicated main story and at each stage were told to simplify it so that even the most obtuse players could know what to do at each stage. (Go to the Keep Depths, dummy!).

I agree with the rest of your post, but "simple and linear" does not have to equal cringeworthy. It almost looks like they exclusively hired a giant ham to write their bosses...

Kulle is the only one with a somewhat natural progression in the conversations (I don't have a problem with his actions at the end being more or less clear from the second you first hear him talk). He feels right, a good mix of insanity, power tripping and trying to seduce the player character. I just didn't get any of that from the other bosses, there's just nothing there. To circle back to the OP: Belial was just... bleh!

take care
Tarabulus


P.S.: As an aside, the only decent reaction to the player character's obvious power comes from the coven cultists. They are terrified, as they should be (among other things, shown through a brilliant little diary entry in Act 2, loved that one).