I work in the industry with game devs on a daily basis. I have it on good authority that 2019 was the target for Sony at least with a PS5. However, there's a good chance it won't make it out until 2020 simply due to the desired hardware not being ready at the price point they need to launch.
In order for Sony to build a PS5 that will deliver true next gen experiences (as opposed to PS4 quality at 4K), they will essentially need to build an APU with 8+ Ryzen/Ryzen2 cores and at least 12TFLOP GPU that is essentially a Vega10 part that is shrunk to fit in the size and power requirements of a console. They will also need at least 24GB of Ram with ~400GB/s bandwidth or higher.
Keep in mind that even these numbers show the continued slowdown of hardware advancements from generation to generation. For example, the memory and memory bandwidth numbers here are only a 3x jump over PS4. We all remember how PS3 to PS4 saw a 16x jump in memory and an 8x jump in men bandwidth. Similarly, even with Zen cores, best case is we see a 2-3x improvement on the CPU side. The days of the 10x jump like from PS2 to PS3 are indeed Loooong gone.
So overall, if you thought the jump to this gen was "small", you haven't seen anything yet