When the Plus-sized iPhones came out, I stopped carrying around my 7" tablet. I've never purchased another 7-8" tablet since.
If a foldable iPhone comes out, I'd probably still keep my 11" iPad Pro, but would likely carry it around much less often. That's assuming I actually buy one. Pricing will be key. As nice as some of those non-Apple foldable phones are, some of them are priced super, super high.
No, a foldable iPhone wouldn't replace an 11" tablet, but in some cases it will be big enough. That's what happened when the Plus-sized iPhones came out. It was still nowhere near the size of a 7" tablet, but it was big enough so that carrying around a 7" tablet seemed like more hassle than it was worth.