The prediction has been fulfilled!
I originally predicted Breath of the Wild (Switch + Wii U combined) would outsell Super Mario Odyssey. Last Wii U sales figures reported were 1.5m for Zelda: BotW. Adding this to the Switch totals yields the table below.
The Legend of Zelda: Breath of the Wild (combined) has outsold Super Mario Odyssey as of June 30, 2019!
This actually happened even faster than I thought. Zelda has some amazing legs. I guess now I'll have to keep watching to see if the Switch only version of Zelda can outsell Odyssey on its own. Breath of the Wild really is an amazing game.
WiiU total is at least 2m because VGC last reported 1.5m but we know from the Japanese sales threads that at least another 500k was sold after that before it stopped charting.