The thing to think about, is how poorly developed, rural places,
without ACs or stable grids, are much more likely to experience heat waves with universally fatal wet-bulb temperatures. Places like parts of India or Bangladesh with high population densities, too.
Further more, I think, we also need to consider how non-survivable conditions to humans, may play out for other animals we depend on. The cows, chicken, sheep and goats? All dead. But consider also the impact on ecosystems, when birds and predators may die in large quantities in a short time - these ecosystems may completely collapse. Great impact on food security in affected regions. Not to mention disease dynamics when pathogens experience body-like conditions outside the body, and all the decay and death around. I am not sure all plants would make it either, since they need evaporation of water from their leaves, too, to transport nutrients from the soil upwards - and with halted transport, may not be able to fight fungi and so on.
Now, this threat may not come as a gradual trend, but may introduce itself as single (first) freak heat-wave killing hundred thousands of people all at once. A whole region may get the message their land is not habitable for humans anymore. Imagine the migration pressure as a consequence. I don't think a poor country could ever properly adapt to frequent non-survivable temperatures sustainably...