I really can't imagine the sequel being released in just 2-3 years. Huge games like this one are taking a long time to be made. Having the basics already done can reduce a lot of time, indeed, but I don't think it's that much, and I also can't imagine crossing 2020 without at least marketing the PS5 for the following year. At least.
I wish the next generation could take longer to come out, but you know how it works. Tech evolves quickly. And we also don't know if they want to work on another IP. Look at The Last of Us, for example. It had Uncharted 4 between the first and second games. It will have been 5 or 6 years between them and the hype is still big.
Time or momentum is certainly not what is gonna kill the hype for a sequel to GoW. Just look at the campaign until GoW4's release. From the little leaked images, to the big E3 reveal and onwards. They know how to build all the hype it needs until they finally release it.
You guys are being overly optimistc if you think it can come in 2020 and still be at least as good as this one is.