I think the GP104 will be coming but the 980Ti will stick around a long while yet.
Hell it was probably planned that way all along if you look at 600/700/900.
Are you thinking that full-fat GP104 will slot in above or below the 980ti?
GM104 (980) is about 25% less perf than the 980ti, according to user benchmarks. (e: 980+30%=ti, ti-25%=980)
Depending on how much of an improvement Pascal is over Maxwell, retaining the 980ti could leave you with two products in almost the same perf ballpark.
If GP104 is 50% faster than GM104, that still only amounts to 13% faster than a 980ti.
GP104 would need to be 73% faster than GM104 to be 30% faster than a 980ti.
A 60% improvement over Maxwell would result in being 20% faster than the ti.
But even then you have a cheap to produce, small chip being priced higher than a costly to produce, large chip.
It's the same story with Polaris and Fiji. It doesn't seem to make much sense to retain the largest chips from the current gen.