Apple always errs on the side of removing old tech. The downside is clear, the upside is that by removing old tech, you help push the industry away from it.
Those carriers will want the business of iPhone customers coming into their country. They will upgrade their systems. Only thing that sucks is the transition period. Sometimes you have to rip the bandaid off and it hurts, but it will probably only hurt for a year or two. If this is that much of an issue, buy an iPhone 7 for cheap and use it in East Africa or wherever for a year or two until the straggling carriers get on board. The cost of an iPhone 7 compared to international travel right now is nothing.
Apple has done this before, with headphone jacks, optical drives, 30-pin connectors, Flash, floppy drives, etc. They were right every time, and every time people were absolutely pissed. And they will eventually be right about this too, and everyone will laugh about how cute and quaint it seems to carry around a series of tiny cards to gently place into a wee little tray that slides into your phone to connect to a network. I'm glad they're killing off SIM cards forever. Apple has the market share to get it done. They've waited long enough having eSIM with trays for 5 years. This one deserves a big fat FINALLY.