It is and you can buy them, but you pay a significant premium for them.

IIRC Cavendish is supposed to be more resilient to the fungi than Gros Michael is, but it’s not immune. The fungi mostly exists underground so it’s difficult, if not impossible to remove from the land once it’s “infected”… And it takes decades to clear naturally once the trees are removed.

The good thing here is that we already have Gros Michael and AFAIK, Cavendish seeds in the global seed vault, so we’re not at risk of losing the ability to bring the trees back at some point in the future. We still haven’t lost them, as you mentioned, there’s still small batches being grown.

IMO, it’s all a bit sad, since apparently Gros Michael is so much tastier, and there’s a shrinking number of people alive who are old enough to remember what they tasted like at all… So without investing in buying some from one of the small batch plantations still growing them, very soon, all but those that specifically went out of their way to try them, will have no idea what they taste like.

I’m not old enough to remember what they taste like (if they even existed as an option in the grocery when I was born at all, which I’m not sure about). I’ll probably never know.