One point of the YAML configuration is to (hopefully) create a situation where a person that just wants to run applications doesn’t have to write any code. Instead they alter YAML configuration to get their task accomplished.
It’s true that Fortran has made a lot of recent advances with object-oriented style programming concepts, not as much in the generic (template in C++) style. C++ is still a clear leader (at this time) in the maturity and available library constructs (ie, C++ STL library) thus leading to the choice of C++ for JEDI.
Depends on your involvement with the JEDI project. If you only ever execute JEDI, you will only ever need YAML files.