I think I used the wrong term for what I meant, so let me clarify now that I've looked up the correct terminology:

There is only one real DLC. It's the bridges and ports thing that had been announced together with the game and you could buy both from the start. This adds actual functionality, similar to real DLC for CS1. Again, there's only 1 of those.

There are (paid) "creator packs" that are basically DLC-light. This is also roughly equivalent to what CS1 had, both in terms of price and content "amount", at least from my understanding. Obviously also completely optional, and kinda similar to Paradox games in general. Some hate the model, some like it. I don't think any of them were actually made by colossal order, but by members of the community, and they also get a significant share of the revenue. I don't know what that share is though, or if that information is publicly available somewhere.

Now the third category is what I actually meant: There are Region Packs (10ish? I think?), also made by Colossal Order. These are basically mods like any other mod in the mod portal. They are actually very significant in terms of content and add a large amount of variety for both service buildings (police, clinic, schools, ...) as well as growable buildings (residential, commercial, ...). THESE are what I meant. They could've made them DLC, but didn't. If they wanted to maximize profit like you suggested, why not do that?