Well, full frame currently
is niche, high end and expensive. It is the single defining characteristic of the most expensive cameras. I see no reason for that situation to change, and quite a lot of evidence for smaller formats like APS-C and 4/3rds to grow - smaller, cheaper cameras, ditto lenses with more range, smaller sensors which already deliver more image quality than most people need, and getting better all the time.
The major manufacturers need a premium line as part of their business plans and marketing strategy. High spec and low volume sales dictate a high unit cost. There is no incentive for them to introduce full frame to the mainstream consumer range.
From a customer perspective, there is no evidence that the majority of enthusiasts need the benefits of full frame, are willing to put up with the compromises it entails, or are prepared to pay for it. That is the overwhelming evidence.
I submit that the foregoing stands up to objective scrutiny