If it wasn't reinforced then it's just aggregate and as as the water de-grouts the concrete the resistance over the area will increase relative to the rest of the structure.
This doesn't have to be the rapid process that you make it out to be. There are plenty of spillway "deflectors" exposed to the constant and direct impact of water dropping nearly vertically off smaller dams that have been in place for 100 years or more, and though the concrete in such cases has inevitably been worn, it is by no means failing in cases where the concrete was of good quality to start with. The same goes for concrete rubble that's often found in swift tail-race areas to disperse energy. There is a tremendous range in the durability of what you are calling "grout", the sand/cement matrix in concrete, and one-size-fits-all statements are not likely to be correct, in my opinion.
When high volume fluid, water in this case, scours a disturbance like that for 24 or 36 hours one of two things happens. Either it creates a nice smooth channel or it rips away material looking for a place to create a smooth channel. Concrete isn't that strong when put up against lateral loads and scouring action like that. Great compression strength along the long axis but not so robust at lateral forces (why some buildings have problems during earthquakes).
The problem with this statement is that it's not the concrete slab above the waterfall that will dictate whether or not significant headward erosion takes place. It's more a matter of how durable the bedrock is. The concrete could even be stripped right off the top, but if the bedrock below is sound, there should be little to worry about. The quality of the bedrock is highly variable within this area, as has been talked about
ad infinitum on the main thread, but so far (also re-hashed many times) much stronger rock seems to be taking the brunt of the water's force at this time, now that the highly-weathered rock is mostly gone from the area of concern. As far as the water ripping away material looking for a place to create a smooth channel, I have seen lots of powerful whitewater runs on rivers which show no sign of the jagged bedrock rock of the riverbed becoming smooth, and no indication of perceptible wear, and they've been running for thousands of years. It seems to me that reasonably durable rock (even if it's not as durable as the rock exposed on the kinds of river beds I mentioned) could be expected to tolerate the present stress for the relatively short time that is necessary here, which is until the dry season, when repairs can begin.
I wish I could provide proper links regarding the durability of rock or concrete when subject to such erosive forces, but that would be of little use to me since I don't have the ability to quantify either those forces or the durability of the materials at this site. But then, you are not presenting your ideas in that kind of context either, so I'm just saying there are plenty of places where the condition of existing dams of great age, or significant drops on natural river beds show that what you say here is not universally true.
