I'm aware, if it lasted forever then I'd gladly pay an additional $1000+.

But as you also know, after 2020 then they'll still offer unlimited high-quality photo/video storage, and I think those who don't abuse the offering by storing RAW files and such won't mind the minor quality difference, and those who do will likely want to upgrade their phone.

imho they are telling that a pixel buys you three years of unlimited storage, if you will buy a new one after three years a new unlimited storage period will follow, maybe for another three years.