That is your opinion. I've been lucky enough to taste plenty of other banana strains and there are definitely better banana's than Cavendish. The smaller ones that you generally get in this region, that are really ugly on the outside, taste so much better.
> you've seen a few big banana plantations
More than 'a few'... literally 3 whole countries worth. There is absolutely no shortage of trees and farms. Just to give you an idea of how much I've seen and I didn't even tag all of it... https://imgur.com/a/olSoTXe
I'm not saying that this disease isn't a problem, but I have hard time believing it is a catastrophic problem for all bananas.
If you're going to go into big problems, I'd say that African Swine flu is a much larger issue for this region. I drove through countless 'checkpoints' all over northern VN/Laos where they sprayed down my motorbike wheels with some unknown chemicals. There are literally signs everywhere talking about the issue. Millions of swine have been slaughtered. Usually you see pigs running around freely on the roads, but once you get to a certain point in northern vn, you stop seeing them entirely.