Nintendo works on several Zelda games in same time, for isntance they worked on Zelda ALBW, Zelda WW HD, Zelda MM, Zelda TP HD, Zelda BotW and even spinoffs like Triforce Heroes and Hyrule Warriors, in same time, difference is that now they will work on Zelda games just for one platform. Just for record, new 3D Zelda (same like new 3D Mario) are rumored for 2020. release, Zelda BotW was finish at end of 2016. so with 2020. release we talking about 3-4 years of development, and its expected that Nintendo will reuse engine and some assets from BotW and that will save quite of development time.
I hope this is true. Or even make a brand new IP using BotW assets aswell and release a sequel and a brand new IP in a similar vain. Kinda like GTA and RDR or AC and WD. That would be decent.