So, my question is obviously, what's with the incompetence?

I'm not the dev, and I'm not in game development, so I don't know "first-hand" the technical reasons for why some games update like this. I did read a dev (of another game) post on steam about why their game updates are such large sizes.

"We've got a few larger files in our content, mostly divided into maps, items, data and so on, in categories. This is done for speed of development during the time we're changing a lot everywhere. This means though, that all maps are packed into one file (or few of them) and every time something changes on one map, there's a lot of those huge files that are changing. Those are massive, going from 400MB to 13GB per file.

1. Redownloading

Let's say we changed a few meshes, added a weapon and switched a few sounds out. If we were to go with the redownloading method, that would mean having to redownload 99% of the whole game each time we push an update, even a relatively small one.

2. Incremental

This means that we only update 1MB chunks of each file, only those that changed. It also means that Steam has to repack those files and it resulting sometimes in needing up to 50GB of additional free disk space to update the game."

So I'm guessing the Succubus devs use the "redownloading" method of patching instead of "incremental".

Same here. In that case, they sholuld rate the updates to optional or necessaary. Downloading 30GB with a 4Mbit connection is a pain, and just for some visual gimicks. That sucks! It can't be a Solution to disable updates.
Mabe it is better to avoid Galaxy and download the Files manualy. It sems, here are small patches available.