The presentation of the PS5 will take place in early 2020 and the launch will take place in the second half of the year. More precisely November of 2020. Until then the consoles will receive architecture NAVI 7nm and etc. Now stop dreaming about 20Tflops or 3.8Ghz ... None of this ... At least if SONY wants to launch the console for $ 700 as you said. Which is ridiculous and unlikely.
I will try to summarize about why "12.6TFLOPs" (funny number btw) is still dreaming.
January 2012: AMD reaches 28nm (coming from 40nm). The Radeon HD 7970 reaches 3.8TFLOPs at a launch price of 550$.
October 2013: The Radeon R9 290X reaches 5.6TFLOPs at a launch price of 550$. Around this moment, Xbox One and PS4 are released at 400$ (lets for get Kinect) with 1.3 and 1.84 TFLOPs, being PS4 a console with a well tight price.
June 2015: The Radeon Fury X reaches 8.6TFLOPs at a launch price of 650$.
November 2016: PS4 Pro reaches 16nm, and arrives with 4.2TFLOPs at a launch (and still) price of 400$.
August 2017: The Radeon Vega 64 reaches 12.6TFLOPs at a launch price of 500$.
November 2017: Xbox One X arrives with 6TFLOPs at a launch (and still) price of 500$.
It took almost five years for PS4 Pro to reach the 7970 raw power at a reasonable price. This is what you say it is going to happen: In three years PS5 will reach Vega 64 power. At a reasonable price. Just because of Navi and the 7nm. Just forgetting that they need to work in a new internal architecture for the console, they need a new CPU, Vega was a total and weird failure in terms of consumption, RAM prices are going nuts because of the smartphones, AMD graphics cards prices are going nuts because of the dataminers, and the price of PS4 Pro isn't even including a 4K blu-ray reader.
Ok, it's not just math about the years to be accurate with pricing and the increasing of performance, but it really isn't rocket science to know that PS5 is going to be anyway a expensive platform, and that PS5 is not going much further away than Xbox One X, even if AMDs Navi just results to be a really huge success.