The ironic situation is that those companies that don't have legacy lenses to support, have an advantageous reason to get a FF mirrorless camera out there. I'm thinking of Fuji and Samsung. Not sure if Sony would sell FF sensors to Samsung, or whether Samsung is in a position to make a FF sensor of their own.
While other manufacturers keep mulling this over, Sony keeps refining their products, like the AF on the A6000. There's a term for this: "Analysis paralysis" Rather than make no decision on FF mirrorless, i would think larger companies, like Nikon and Canon, could produce at least one FF mirrorless, along with their numerous OVF FF, and let the customers decide which they want to buy. I think its called capitalism
Let the market decide.