I have an iPad mini 4 that still gets used a lot, but really only for one main purpose - it’s the perfect size and weight for reading ebooks and documents. A regular iPad is a little too big and unwieldy.
The problem is that, for watching video or using it as an alternative to a laptop, the mini’s too small, and I find even the normal iPad is too small, this is where the 13” air or pro are great.
So, I understand why there’s different iPad form factors, and which one you prefer very much comes down to your use cases.
I expect the folding iPhone will be the same - some will like it, some won’t, and ultimately that will come from what people use their iPhone for.