Don't see it happening this time. That will require 8 of the 2nd-best binned chiplets per product.
I suggested on the Ryzen thread as part of a different discussion that arguably the 3950X could hamper the availability of the 3800X, given that for every 3950X available that's 2 3800Xs not sold, so there would be a juggle going on with chiplet stock. Now as Threadripper traditionally has the same, or slightly better, clocks as the top-end desktop Ryzen, a 64 core TR would require 8 of those same chiplets going into the 3950X and 3800X, and therefore for every 64 core TR available that's 8 3800Xs not sold. That's a big chunk out of the inventory for high-volume mainstream parts.
I think for this generation 48 cores is a sufficient uplift over the previous generation, and 48 Zen 2 cores will absolutely destroy anything Intel can even dream of.
That being said, the 2990WX actually doesn't have the highest clocks of the Zen+ family, so AMD could take a bunch of 8 core chiplets that don't clock particularly well (say, 4.3GHz boost) and smash those into a 64 core package.
I think the notion of Threadripper requiring the best silicon is no longer relevant, I've regularly suggested a 32 core, 4.6GHz boost Threadripper can be made up of comparatively "junk" chiplets yet is a monster greater than the sum of its parts.