I'll just present my points quickly to help the argument, and then get the hell out of here:

1. The Xbox One and PS4 don't add much to the market either. PS4 has an 110-120k baseline as of late, Xbox One and Wii U have been holding close to 60k for a while. That's a lot closer than the 5th, 6th, and 7th gens.

2. That is not a good reason to doubt it can do 1.5 million this holiday. In 2013, Wii U did 1.6 million throughout November and December with more places having stock (Maybe) and Super Mario 3D World. 

This year, Mario Kart 8 pushed the baseline up for at least the rest of the year, Smash Bros. has more selling power than 3D World, the stock problem isn't large enough to take a huge effect, and Wii U is up YOY (Even with the new competition), so I have no clue why you think Wii U can't reach 1.5 million units this holiday.

If that were true it wouldn't have fallen on its face right out of the gate like it did.  The problem with this notion is that the wii virtually disappeared off the face of the earth in 2010 leaving sony and microsoft to fight over the market and it seems to have hurt nintendo's brand recognition.  Add poor marketing, the bad choice in name, underestimating HD development time and costs, inferior specs with a bad price point for those specs, well, hindsight and all that.

The Xbone and PS4 add more than the WiiU simply on terms of power allowing for innovation, with the PS4 being on top. Everyone I know who owns a PS4 is impressed with the exclusives, and those with the xbone are dissapointed. People always want a jump in gameplay and graphics from one gen to the next along with new IPs - the wiiU is the least favourable option when it comes to either of those points.

The WiiU was up against brand new consoles that always take a few months to pick up after the initial burst of sales. Now people have more of an idea on what is on offer in the market place and I strongly doubt many gamers who have not invested into a console will opt for the WiiU over the other 2. As I said, why would they? Unless you want to play Nintendo games, the wiiU offers nothing, and most Nintendo fans already own the WiiU.

MK8 pushing sales up from now to Dec? Really? It has already dropped to 60k. In a month it will probably be back down to ~35k as it has been since Christmas.

I definitely think it can as long as Nintendo keeps pushing out games we want.

1. More powerful hardware doesn't cause innovation anymore. Sure, there can be more content in the game, but what kind of new gameplay can you come up up with that couldn't be scaled down on the PS360?

2. No one is opting for the Wii U over the other two, that isn't the point of the console. It's a secondary console. You get PS4/XBO/PC for Third Party games and their exclusives, and a Wii U if you like Nintendo games.

3. If most Nintendo fans owned a Wii U, that would mean more than half of all Wii U's to be sold have already been sold. Where do you get that fact from? Do you really expect Wii U to sell less than 13.4 million units?

4. I'll bet on that one. I say Wii U won't get to 35k until at least 2015.

1. Tons. Just take a look at the new 8th gen engines and what they can do, or just ask anyone who owns Second Son and how that plays compared to your 7th gen titles. I think the ultimate game to show power is needed for innovation is Star Citizen with its increible physics engine, how the ships are made, the damage model etc. Such a game simply can not work on platforms with limited power.

2. Well that only applies if a) people arer not entertained by their first console b) they have the money for two. I'm not saying it doesn't, it just isn't a great buisness model to have.

3. Yes. Pretty much that. Just look at surveys and ask people what they play. Nintendo failed to bring about a new generation of fans. The bulk are older N64 owners. Since then the GC failed to bring about the same scale of new Nintendo fans, and the Wii was mainly a party console and as such, had huge sales but did not have holding power over its customers.

4. Just look at the past sales and bumps. This one is no different.

1. I don't see how that means sales will be around 8 million below GameCube. Wii U will pass 13.4 million by the end of 2015.

2.The only other game that was able to boost the Wii U past 50k without the holiday season was Wind Waker HD. 

Wind Waker HD:

The amount of consoles sold before it went back to under 35k was 163k. It had 3 weeks above 35k.

About to edit...

Mario Kart 8:

The amount of units Wii U sold without going back to under 35k is 607k and counting. So far, Wii U has had 8 weeks above 35k.

Do you see the difference?

Edit: The only other boost is the holiday season, which dropped so much in January that July this month will pass it with the next update.

1. *facepalm* The GC was about when Nintendo had a better reputation, was more popular, and the console was on par with the others. None of those are the case now. Now before anyone runs out screaming saying Nintendo is still number 1 popular etc, it isn't. 8 year olds want CoD and MC, not Mario. Times change and Nintendo have not picked up on that.

2. Maybe because MK8 is a popular and new game for the WiiU. Any HD reboot is never a huge seller. Quite a few people will have been holding off until one of the new mega titles came out, and MK8 would tick that box so those people have now bought the WiiU. However I suspect that those waiting for SSB or Metroid would have bought a WiiU by now rather than waiting years.

I might have thought it was possible before Mario Kart came out and we had that amazing Nintendo dominated E3, but now I don't think 20-40 million is that far-fetched anymore.

1) As others have pointed out, the PS3 and 360 also started slow.
2) PS4 and One are both going to be stuck in multiplatform hell, and exclusives are going to be far between. PS4's big games this year afterall is a remaster of a game that came out a year ago.... If this becomes a pattern, I think people's opinions of the console will drop sharply.
3) Nintendo's downsides are pretty much overhyped... When people give it an honest chance, they have pretty much universally given it praise. I think as the gen goes on, we're going to see Nintendo cleaning up a lot of these misconceptions and that will naturally increase sales.

Regarding the second point: How is getting almost every multiplat game out there hell? And do you realize that the crushing majority of PS and Xbox consoles are bought for multiplats? The exclusives are just the sprinkles on the cake for most and looking at 2015 it seem that at least the PS4 (the X1 probably as well) is going to have more then enough sprinkles.
Also regarding 3: That the Wii U is almost exclusively a Nintendo game machine (meaning there are hardly any non-Nintendo games) is not a misconception, so if you don't care for Nintendo then you won't care for the Wii U. And to change that up they'd need third parties.