It would still be interesting to know what you think you can achieve with this approach, i.e. in a direct comparison with Flask (because as you wrote it's quite similar in the featureset) what do you think is the advantage of your approach? Performance is the only justification you give on the page, so I'd be curious about this.
Other than that I guess it's useful if you want to display data from a C / C++ program without going through any bindings. When it comes to that, maybe it would be great to get some automatic HTML display helpers for data structures common in C, like Arrays of numerical values or pointers to sequences of structs. For example in Python it's really fun to just use something like [1] together with Flask and get interfaces to your data without doing any template programming.
[1] https://github.com/softvar/json2html