The upcoming AMD Navi GPUs such as the Radeon RX 5700 will use a hybrid GCN-RDNA architecture according to information obtained from AMD by Swedish outlet, SweClockers. Apparently, AMD feels that GCN is a mature architecture that has been optimized well for modern games and therefore, will find place alongside RDNA in Navi 10 GPUs. AMD is expected to completely transition to RDNA with Navi 20 sometime in 2020.
https://www.notebookcheck.net/Navi-10-based-AMD-Radeon-RX-5700-still-has-elements-of-GCN-pure-RDNA-coming-only-in-2020.422870.0.html
That's baseless speculation when amd developers already confirmed that rdna is a new microarchitecture that makes use of gcn isa.