I feel like people are underestimating how much is already done for SL. That combined with no huge new systems means they're going to release in 2020, even disregarding the fact the pre-order says it'll be on or before Dec 31st, 2020.
Covenants are essentially the Class Order system (as it's supposedly not another rep grind), there's no new AP system, no new class/races to tune abilities/racials, et c; even Torghast is going to be the 8.3 portal-thingies' that name escapes me currently, which is already being tested on PTR + will have plenty of feedback once it goes live. Granted there will be the Legendary forging system + bringing back old abilities and fine tuning specs back into class and new tweaks to old systems, but that's just it-- tweaks to existing systems.
Of course that's assuming they don't make the tweak and the entire system explodes.