1. Third parties are biased against Nintendo
"We do not believe that there is an audience for our games." and forego giving support to Nintendo. This is an unsolvable problem for Nintendo, because the only realistic solutions would be to either pay off third parties for ports (thus ruining profitability) or build an audience for such games themselves which would lead into another commonly used excuse ("Only Nintendo games sell on Nintendo systems."), because consumers' expectations for quality and polish would be raised to levels that third parties are unable/unwilling to match.
2. People buy Nintendo systems in order to play the best games
There's no sales data (neither current or historic) that suggests that multiplatform games are in high demand on Nintendo platforms. Any list of best-selling games will have first party software at the top, followed by exclusive third party games, followed by multiplats. Therefore it's preposterous to suggest that Nintendo should build their console around the games with the least demand; the result would be an expensive system that compromises the values that consumers are looking for.
3. Xbox One
With the Xbox One we have a system that gets all of the multiplatform support and yet it's still tanking hard. Who believes that the Xbox One could have beat the PS4 on a global level, if it launched at the same price, had no Kinect, was equally powerful and had none of the DRM nightmare before its launch? Continental Europe and Japan would have still picked the PS4 in much higher numbers than Microsoft's box. After all, the Xbox 360 failed to make significant inroads in the seventh generation despite having all the multiplatform games (and usually slightly better) and a clear price advantage. Therefore it's not outrageous to say that even a 100 Euro price advantage wouldn't threaten PlayStation. It's clearly an uphill battle for Microsoft and about the only reason why the 360 was competitive at all was the tremendous blunder that Sony committed with the PS3.
I think your analysis is fair but short quite sighted.
The visible bias against Nintendo is BECAUSE THEY DON'T design their systems with 3rd parties in mind. Even sony suffered from this somewhat with the PS3 through difficult architecture many ports were inferior and the 360 was designated as lead platform. Obviously when you had a commercial flop like the Wii U coupled with hardware disaligned with your developers vision its easy to give the platform a miss.
Just to break it down, the bias exist in 2 parts as far as im aware:
Percepton of Audience: You can't underestimate the extent to which hardware shapes the audience a system draws initially. Obviously turning up at E3 and trying sell a system which is only marginally more powerfu then last gen systems sets you apart from the core gamers who play these viseral, realistic and graphic games. The same gamers for whom graphics is important and love M rated content. Infact it somewhat distances you from the entire userbase of PS3/360... If those same gamers that you make games for are the audience who is being ignored, surely that immediately makes the platform in question low on your priority. Power aside, the same applies to your online infrastructure; Nintendo isn't going to attract people who care about online experience, if they themselves don't care about online experiences. Theres a reason why COD sold 10x better on ps3 verus the wii despite the wii having the bigger userbase; if you were interested in that kind of game you would likely have invested in a HD system with a better online environment...
Compromise: This is a lessor issue but i don't think many devs are interested in compromising to fit the wii U's power restraints. If the system had a huge userbase, I'm sure many would change their tune. But the likes of Bethesda, Bioware, rockstar, Square Enix probably wouldn't with their flagship titles.
Both things are solvable. Make a console with the modern core market in mind. Nintendo do not, hence why there are refered to as out of touch.
Multiplatform titles aren't in demand on Nintendo platforms because they more often then not, superior on microsoft and sonys platform- normally a fault of Nintendo's hardware. Even if FFXV came out on the Wii U, I think most fans would want to experience it on the PS4.
The fact that data shows that most multiplatform titles fail on Nintendo hardware is probably why 3 of Nintendo's last 4 home consoles have either flopped or underperformed. Its a problem. Nintendo need to address it, not just shrug it off.
Invalid. Microsoft went from selling 20m one generation to selling 80m and still counting. They couldn't have done that without 3rd party support. The Xbox One is underperforming as you clearly illustrated because of numerous mistakes on microsoft behalf as well as a lack of compelling exclusives. Once next gen exclusives arrive and 360 owners are forced to buy either the PS4/XO, it sales will most likely become more competetive... Even if they don't the system has still sold at a faster rate then the Wii U, not to mention all the money they make from strong 3rd party sales (royalty money).
In short, Nintendo should make a system which can appeal to the mass of video game buying public (powerfull, not too expensive) as well as their core- naturally appealling to most 3rd parties and the games they craft. Their 1st party games have more pulling power then anything sony has and probably microsoft too. The biggest issue is the childish image they've been assigned in the last decade.
They will never achieve sales of 20m+ just looking at their niche fans who chatter about rumours of a new star fox every year. They can try the blue ocean approach but next time they need a better gimmick then the gamepad. The safest thing they can do is pay attention to what 3rd parties and gamers want.