Simplicity is the key for the end user
Hi Steve,
you are right and I understand a little bit that you are wondering why there was a change from ini to yaml.
This change was not done solely because yaml is so f* * awesome, but to develop a better agent.
The CMK team works hard to keep CMK to make it as compatible as possible to any system out there (just think of >1700 checks which come by default!) and at the same time to make almost everything configurable by WATO. If you are using Checkmk since 7 years you know best that the product constantly evolved. This cannot be achieved by always only adding new features and keeping anything else as it is. Sometimes you also have to “break” with former design decisions to make the next big advance. (If you have ever worked with large INI config files of NSclient++, you know why YAML exists. 
)
Reading/editing YAML files is not that hard as you state, just give it a try - and always remember (also with a little bit of respect to the developers in mind): it was all done to deliver a better product. The YAML you are talking about is perhaps the remaining 0.01% of “complexity” which cannot be hidden by a nice GUI. The other 99.99% is what should really make you happy 
I guess there is nothing in the IT world which does not require a little bit of readiness to learn something new from time to time. I even like that 
PS:
Even most technical people like simplicity…
Yes… but they also understand easily why some things must change whilst technology is proceeding.