The way things have been for RTG I bet Navi's close to two years away still. I don't know what to make of Vega, It's not worth what they want so the smart money is on waiting it out, Pascal's over a year old now so it shouldn't be too long till we see Volta in some form.
I don't think they are going to throw everything away with Navi and start all over again with a completely new architecture - I suspect first version of Navi will be largely shrunk Vega with some architectural changes to lay the ground work for a further revision towards more scalable architectures (both processing and storage wise) in the future.