We are talking about a configuration language here, which is never about markups. Also, most of the so-called whitespace problem in Ansible or k8 configs comes from embedding config that's not YAML. In these cases you need a path or URI for the processor to resolve before applying, not inverting the entire configuration language to cater to 1 uncommon use case.
The point of my original comment was precisely that SGML was optimized for text documents. I agree that adopting it for configs was a mistake, but the complaint that "IBM lawyer who invented the SGML syntax had never heard of S-expression" doesn't make any sense in that context.