Let's go through each of your doom and gloom points..
1. You must have a different definition of "wonderful" than I do. I don't see weekly sales of 125k and 89k as "wonderful." These consoles are clearly in decline and are showing their age. They are dying like the Wii. The difference is their death is being prolonged a bit because Sony and MS have kept quiet on their next consoles and third parties continue to crank out games to keep them afloat. Sure they are better than the Wii, but that is primarily because Nintendo and third parties have all but abandoned the console, and Nintendo has annouced the Wii's predecessor, making people reluctant to pick up the console that will very shortly be replaced. What big games remain for the HD consoles that will carry momentum? You have the next call of Duty and Halo, but that's about it. The only reason these console still put up decent numbers is because Sony and MS are riding out these consoles as long as they can, and third parties continue to make games for them. This can only last so long. They are already beginning to seem dated and once Wii U is released, they will only seem moreso. The sales of the current HD consoles will have little effect on the Wii U market, just as they did on the Wii market, as they largely are after different markets. A few ports from these consoles to the Wii U doesn't change this.
2. Hardcore gamers mean little in the grand scheme of things. They repressent a small portion of the overall gaming audience. The Wii was almost universally hated by them and yet the Wii is still on pace to sell 100 million units. The Wii U will almost certainly get more of their support with their more respectable horsepower and seemingly better third party support. Also, who's to say Sony and MS cannot lose this audience? MS in particular seems to be making it clear more and more with each E3 showing that their primary interest lies with the casual Kinect audience, not with the hardcore.
3. This is all speculation until an actual price is announced, but anyway, Nintendo would not possibly price their next console for more than $300 if they are in their right mind. But even if they decide to pull a Sony and price their console for an unreasonable $400 or more, just remember, Sony initially priced their PS3 at the insane price of $599, and they've still managed to sell 65 million units. The impact of price is overrated. People won't care much about price, as long as there are the games to warrant the investment.
4. Again, you are putting too much importance on the so called hardcore market. From what we've seen and heard, the power will be "good enough," anyway. I can't see MS and Sony's next consoles giving a big enough leap in power that it will be that noticable to the human eye, at least not to the point of warranting the purchase of a $400+ console. We are close enough to the point of photorealism that there is simply not much room left for vast improvement. Now, Wii U will almost certainly be the least powerful of the next generation of consoles, but that means little when talking about sales. If anything, hisotry has shown that the least powerful console almost always becomes the most successful.
5. It is pointless to talk about future competitors when we know absolutely NOTHING about their next consoles. Sony and MS may not even release future consoles, or they may bomb miserably for all we know.
6. Again, you are placing far too much importance in gaming demographics - in this case, the mythological "casual" gamers. While there has been a major increase in tablet gaming, there is no evidence to indicate that this market will take away from Nintendo, or that it was even the same market that bought the Wii for that matter. Even if it is largely the same market, there is no reason to conclude that these people will shift exclusively to tablets. There will ALWAYS be a large market out there that prefers traditional gaming, that is - serious games on large TVs at home with controllers in hand. Tablets are no replacement for home consoles. I don't even believe they are sufficient replacements for handheld consoles. Like all of these points, It will boil down to the games. If the quality games are there, they will buy a Wii U.
While I don't think Wii U will realistically achieve Wii success, I think it'll sell respectable numbers like the SNES did (for its time), and have games that appeal to a wide variety of gamers. Let's not forget, this is a console that will have 2D and 3D Mario, Smash Brothers, Zelda, Call of Duty (probably), Wii Fit, and Dragon Quest all on one platorm. I think it'll do just fine..
