Retail demand will ultimately decide what is available and what isn't. If the (few) 26" bikes that remain available are flying out the door then there will always be some manufacturers who will step up and fill that demand. If not, then they won't.
I remember ten years ago tech industry analysts were predicting that high performance personal computers were a thing of the past because everyone would switch to cloud-based apps. Hasn't happened yet. Not saying it won't eventually, just that long term trends don't always take root when driven solely by corporate expedience.

If Mark Twain were alive today he might say something like, "The reports of the demise of 26" MTBs have been greatly exaggerated"