You can deduce some things from these sales trends, but certainly not any certainty that Zelda U is "destined to flop hard."
So far, sales of home console Zelda games have hit a high point every decade or so. 1986 with the original, 1998 with Ocarina, 2006 with Twilight Princess (highest point yet if you combine GC/Wii versions). Based on that evidence, and the possibility of Zelda U launching on NX too, you could just as easily argue 2016 will see the franchise hit new heights. Each peak point for the series also presented particular conditions that seem to elevate Zelda away from its more typical ~4 million range. Obviously the NES title introduced the franchise, Ocarina was (and remains) the highest rated game of all time, a return for the franchise after a 4 year gap and the series transition into 3D, and Twilight Princess was a cross-platform release and a launch title for one of the fastest selling consoles ever. These high points figure in the 6.5 to 7.6 million range, or up to 8.8m if you count both versions of TP together.
The more certain trend you can identify with home console Zelda, is that outside of remakes, and outside of Twilight Princess's GC version (overshadowed by the immensely popular Wii with TP as launch title), a new home console Zelda can sell anywhere in the 3 to 7 million range, with sales in the 4-5 million range the most common, despite the disparity in userbases on Nintendo home consoles. Sales for Adventure of Link, Link to the Past, Wind Waker and Skyward Sword are all in the 4 to 4.6 million range, despite userbases of ~60m, ~49m, ~22m, and ~100m respectively. You can also argue that if a Nintendo home console sees two Zelda games, the latter title will sell fewer copies than the first Zelda title. This hypothesis has not been tested against remakes, and I see no reason why we can assume Zelda U will sell less than HD remasters.
On portables, this argument might hold more ground. 'New' portable Zelda titles sell in the 1.4 to 5.1 million range, whilst remakes have sold in the 2.2 to 3.8 million range. I should point out that Phantom Hourglass stands alone at the top-end of portable Zelda sales, with the more typical range being 2-4 million for portable Zelda. Importantly, every portable Zelda remake bar Majora's Mask 3D (the second remake on 3DS, and the worst selling home console Zelda) have outsold new portable Zeldas on their system. This trend, of remakes outselling new Zelda titles, is restricted to portable Zelda titles, which have a lower range of sales than home console Zelda titles, with both their high point and their low points being below the series' typical range on home consoles. Further more, there are several conditions that one should take into account when considering the success of Zelda remakes on portables. Two of the remakes were of two 'fan favourite' games, LttP and OOT, which are revered widely in the gaming community. The sales of Link's Awakening DX are also quite close to that of both Oracle games (300k), suggesting a fairly stable Zelda fanbase on portable systems at the time (1999-2001).
There is no evidence, as of yet, that home console remakes damage the sales of new home console Zelda titles, because we have yet to see a new Zelda title launch after a remake on any home console. The trend is prevalent on portables, but even then, portable Zelda hits a lower range than home console Zelda, suggesting the series is stronger on home consoles comparative to portables. Based on the evidence I've seen, I would say that a Wii U only Zelda U would sell 4 to 4.5 million copies, based on the strength of Nintendo's top tier franchises on Wii U and historic figures for new home console Zelda titles. Cross-platform Zelda is difficult to predict, but, if it does happen, it may be another "once in a decade-ish" event for the series. A strong NX launch could carry Zelda U's total sales into the ~7 million range the franchises' top titles have experienced, and I would assume similarly to TP, the bulk of these sales would come from the new system. Based on what we do know so far, Zelda U also occupies some of the territory commanded by the franchises' previous high points: a large overworld, and Link on horseback. Tenuous, but they fit the more 'classic' Zelda image etched into mainstream gaming culture.
TLDR: Portable Zelda shows a trend of remakes selling better than new Zelda titles on the same format, home console Zelda shows no such trends and suggests Zelda Wii U could sell anywhere in the 3-7 m range depending on cross-platform possibility (if successful NX launch title, higher sales likely), with single format Zelda U most likely to sell 4 to 4.5 million copies based on historical precedent regardless of userbase size and strong attach rates for top Nintendo titles on Wii U.
