For the last 2 major Zelda titles (Skywatd Sword and Breath of the Wild) weren't there a few years between the initial announcement and the eventual release?
I mean, I'd love to see BotW 2 sooner rather than later but I'd be amazed if it comes out in 2020.
If it's running the same engine, then I'd say the next Zelda should be out sooner than most people expect. They already have a base for the game from BotW, all they need to do now is retrofit it with new assets and story and you should have a late 2020 release at the earliest.
Nearly zero chance of that happening without price cut and I'm not counting on it happening either in the long-term. By the end of this year, the vast majority of Nintendo's leading game franchises will have released thus the Switch will likely end up experiencing sagging sales next year ...
Then they can delve into their lower tier properties and focus on more unique and niche games for 2020 as well.