That is where training comes in, if you train with an M9 and on the range your required to draw from a holster you will get use to flipping the safety off and it becomes second nature.
I have always thought the very same way regarding those two safety types.The way I approach the slide safety v the frame safety is to more my thumb forward, then down. This motion will hit either type safety, if they're engaged.
Wasn't it the us military that required a slide mounted decocker in the first place? Also if you know how the safety / decocker works you can see it would be impossible to mount it anywhere else.
but the slide mounted ones we designed for ...I don't know what...
If one wants a Beretta 92, but not the hammer-block/drop safety, Beretta makes the 92D