Greg Reid wrote ...
...So why has this idea taken so long?



It hasn't.

Hartman & Biermann investigated & tested such props back in the
1930's. While they do indeed gain a bit in peak thrust, they lose
quite a bit of thrust at take-off. Minor thrust improvements at
cruise have relatively negligible benefits and major losses in thrust
at take-off have significant negative effects, hence such planforms
were shelved for almost all applications. Reference NACA Technical
Report 643.

Daniel