Oh boy, this will ruffle some feathers of it's true. Ever since we've first heard about Navi, most computer enthusiasts and journalists assumed it was a high-end GPU. FUD Zilla claims they've been sitting on a story for a while now that Navi is not a high-end GPU, but a replacement for Polaris.
According to Fuad Abazovic's sources, AMD will not have any new high-end GPUs until at least the end of 2019. Basically, FUD Zilla claims Navi will be a GPU with a small die area, and a performance level that's roughly on-par with the Radeon RX Vega series.
Radeon RX 580 / 570 definitely needs an update as this Polaris based architecture was simply a slight improvement over the Radeon RX 480 generation based on GCN 4 and Polaris core. The new Navi 7nm mainstream chip would bring much needed advanced to stay competitive and add some Ray Tracing acceleration elements in the chip too.
The real Vega successor isn't anticipated until sometime in 2020 and that puts AMD's Radeon Technologies Group in a very awkward position if these rumors are indeed true. At the moment, there are no details about what the future high-end product is. Perhaps that will be a multi-chip module (MCM) with two or more Navi chips.