One of the major use-cases for YAML is as a host language for DSLs, essentially basic programming languages for specific use-cases. TOML isn't as good there, as these languages often need deep nesting of data structures. GitHub Actions is the obvious example.
Yeah, toml isn't great for that. It's greatly preferable to yaml for human-readable config. Json is better as a serialisation format.
DSLs are bit vexed, not least because they're often horrible in themselves - cruft-accumulating hacks with no tooling, blurring the distinction between code and config often as a workaround for unwieldy dev processes. That's certainly not true of all DSLs, though (can't speak for Github Actions) and there is a need for a simple common base syntax/parser for them. I don't think yaml is a good solution.
Being a cantankerous old git, I tend to think the problem of a common base syntax/parser for DSLs was solved 70 years ago: s-expressions/lisp.
I'm very iffy on DSLs in general. One that I'm using at the moment has foreach loops but no if statements! And... all the inputs must be specified in a YAML file!
toml