I would be stunned if we don't get D4 eventually. Although eventually may be half a decade or a decade from now. D3 sold over a billion dollars worth of software. When they can't squeeze any more money out of class packs and the like, they will eventually make a sequel.
Personally, I think the future of D3 is just the low effort we've seen in '16 and '17, and whether we ever see any more class packs will depend on how well the Necro pack sells. Personally, I hope its a flop because the sooner they close the chapter on 3, the sooner they can really push development of 4.