I'm always in favor of small config files, but sometimes for larger projects that's just not possible (or desirable). TOML has always been about being as simple as possible while still solving a wide range of problems. But I think of it as the 80% solution, where 20% of projects might need something more powerful to solve their more complex needs, and that's ok. We still need a super simple config file format for the 80%.
As for adoption, it takes a long time for something like TOML to be adopted. Inertia is powerful in the development space. I also haven't had as much time as I would like to push TOML forward and evangelize it. My hope is that at some point there will be a tipping point and you'll see most projects start with a TOML config and only change to something else when TOML can't meet the project's needs.
Yes I've got a project and config that I tried to stuff into TOML and even make my own DSL for it, but I haven't gotten anything satisfactory still. I've settled on some semiugly yaml for now until I try again
May be the config shouldn't be large and complex in the first place?
My biggest question is, why hasn't TOML used much more widely or becoming de facto standard? Am I missing something obvious?