I'm so confused about the heat. The idea that 2D engines are orders of magnitude less complex than 3D engines is ubiquitous and frequently stated by many reputable game devs. Complex game engines will obviously have more complex code. Factorio in particular is certainly pushing the envelope there - with or without a 3rd party rendering engine. A better example would be 2D RPGs.

The distinction is in quantity and types of assets. In 2D you usually have some bitmap and vector assets, collision data, animations, and scene structures to tie it all together. In 3D you have all of those things plus the texture/model/material distinctions, lights, skybox, and all the other details that come in as you add more rendering capabilities.

However, you can absolutely make 2D games that need lots of kinds of assets. RPGs happen to be one of them, in fact: The incidental details of adding inventory, NPCs, abilities, dialogue, etc. does add up. Every little message and description, every item's properties. You can ship a game by hardcoding much of it, but that's not going to scale with any substantial team. You need real data management, build processes, etc. Where assets interact with each other you get incidental complexity of the Dwarf Fortress bug kind, so where you can, you add static checks. The engine is built in tandem with the asset pipeline, in effect.

So - whether it's 2D or 3D, what actually matters is the assets. That's why even in the early 80's you had vector games with 3D effects in them; they just took approaches that simplified the assets and the resulting scenes.

Because that's not what you were saying: You literally said a 2D engine barely deserved the name, and that it was usually not worth the hassle to use one rather than writing your own.

This reeks of moving the goalposts.