Can't really comment whether it would be really WORTH upgrading to a NAVI GPU, when it lands. We need to wait for benchmarks, but I'm not very optimistic with AMD. I'm just to curious to know, whether AMD implements it's own Ray tracing on the hardware level, with these new GPUs.
But you are correct. According to the roadmap, NAVI 20 is going to land next year, 2020. These might be high-end NAVI GPU offerings, though this is just based on pure speculation for now. Also, I think NAVI would be the last AMD GPU to be based on the GCN architecture though (refined). In 2021 we might see a completely new arch, rumored as ARCTURUS, (most probably on VLIW2, or as AMD calls it SUPER-SIMD). This is where things might change for AMD.
IMO, on a slight off topic note, I think APU is where AMD can try to put a strong mark on the market. They will have a hard time capturing the high-end dGPU market as evident from their recent trend. I could be wrong though.
I'm still skeptical about NAVI though. The Radeon VII wasn't a very worthy contender either, at least in my opinion. It looks like AMD really wanted to compete with NVidia's RTX 2080, but didn't have much choice apart from re-branding and releasing a cut-down variant of their current MI50 Instinct compute card. Seems like a desperate move from AMD.
This R7 wasn't meant to be an actual gaming card to begin with, because AMD had plans to target the compute/HPC segment as well. They didn't have much choice either, so they just made some changes to the existing GCN architecture on a refined process Node though, giving us this R7 GPU.
This is also evident from the FP64 performance of this R7 GPU, which sits around (3.5 TFLOPs). AMD had a change of heart, deciding that their Radeon VII users deserved a little more FP64 performance from their new gaming flagship, making the GPU more appealing to professional users as well, while maintaining the performance advantages of their Radeon Instinct lineup.
But for gaming FP64 is irrelevant though. The inclusion of 4 existing HBM2 memory stacks also actually made this card to be priced in a higher bracket, as compared to the Vega 64 and similar cards.