1. Use a declarative-style format for config, which is fine, I guess; 2. ... and when that inevitably fails, you... use a programming language?
Some people -- not all, but some -- may prefer to skip the first step and go straight for the second option. Particularly if it's a complex thing indeed like a build system and not, y'know, configuring your blog.
The problem is shitty config languages. I wouldn't group xaml in the same category as dhall for example.
Language is what people use, and "alot" is close to critical mass, so you'll outlive this species of pedantry. I just like the cartoon.
It also means your plugin system doesn't have to choose an "easy" or "simple" language. You could use wasm if you wanted to and allow just about everything under the sun. Although, Lua is used for Roblox games so, maybe it's complex enough.
Now it's all a single simple lua function and 3x as fast... I think this change is great.
Maybe it fails because you are trying to solve a problem with a configuration when it should be solved by a function implemented in the program you are trying to use.
This assumption is doing a lot of work. Surely the question was precisely why it would fail?