I wouldn't say it's not possible that the release Vega replacements first in 2H 2019 but we've had not even a rumour of that being the case
Quoi? Navi is the Vega replacement, always has been. Just because the rumour mill has said that Navi is pitched at the midrange doesn't automatically mean it's replacing Polaris.
RX Vega can be "pitched" as a flagship card all AMD want, but it only has upper midrange performance, being as it trades blows with the 1070 and 1080. RX Navi coming out with better-than-1080 performance (but probably not quite 1080 Ti) still makes it midrange given the existence of the RTX, but that doesn't replace the Polaris territory segment.
That's why the 12nm Polaris 30 rumour made sense in the bigger picture. Now yes I was a bit wild with my pipe dream post earlier in this thread, but market segmentation is such that AMD couldn't get a Polaris product into the upper midrange bracket, and couldn't strip down a Navi 10 enough to be a mid to lower tier product.
So we get a new 12nm Polaris 30 to refresh the RX500 line and 7nm Navi 10 replace the RX Vega line.
Of course, factor in "Big" Navi 20 and all of the above goes out the window; Navi 10 is the little one to replace Polaris, Navi 20 replaces Vega. Would this mean Arcturus gaming cards wait until 2021? "Next Gen" was 2020 on the roadmap, but that's going to be a pro card first surely.