Just Dance is the exception, not the norm. Just Dance, shovelware crap that comes out every year with minimal effort put in, is one thing; Mario, Zelda, Xenoblade, and Nintendo’s other franchises is another.
The fact still remains that Nintendo’s resources were limited and if they had spent any more of them on developing games for the Wii post-2011, those are less games coming out on 3DS, let alone the Wii U.
Hindsight: 20-20 The Wii U was going to flop regardless and there was nothing that was going to save it. The 3DS on the other hand? That could still have been salvaged and it was. If you took some of the early 3DS games like NSMB2, FE: Awakening, KI: Uprising, Tomodachi Life, 3D Land, and Sticker Star and put them on the Wii. The 3DS was already struggling enough to gain traction. How much more fucked would that system have been if it lost some of those titles?
Not only was the 3DS going to sell considerably less. Some of those titles I listed would have sold considerably less on the Wii than they did on 3DS. Galaxy 2 had already sold around 5 million less than Galaxy 1, and that was when Wii sales were still relatively healthy. So what would 3D Land have done on the Wii at a time when the Wii was all but dead post-2011? How about Uprising? Does Sticker Star still do 2.5 million?
What about Fire Emblem? Awakening was supposed to be the last game in the series until it blew sales expectations out of the water, being on the 3DS when its sales and momentum was at its peak, was one reason why. Does Awakening still reach those levels on a dead system where its last entry, Radiant Dawn, didn’t even reach .50 million when the Wii was at its early peak? Congratulations, Rol. In your scenario, Fire Emblem is dead.
The only game out of that bunch that I could see doing better would have been NSMB 2.
But one game doesn’t justify the underachieving performances of other games and especially underachieving hardware. 2011-2023 was the one time where Nintendo posted operating losses, how much steeper would those losses have been if they went down the route of giving more support to a system in its way out?
The writing was on the wall. It was time to move on.
I already mentioned that Nintendo rushed out successors to the DS and Wii to beat Sony to the punch. That ties directly into killing off the Wii too early which in turn ties directly into your question how much steeper Nintendo's losses would have been if they had kept supporting the Wii. Their losses would have been mitigated by doing that, because a hypothetical NSMB Wii 2 selling only 10m copies (very lowballed figure) would have still brought in more than $250m in profits for Nintendo.
Fiscal year ending March 2011: https://www.nintendo.co.jp/ir/pdf/2011/110425e.pdf
Wii software shipped 171.26m units. To put this into context, Switch shipped 168.72m units of software in its most recently completed fiscal year ending March 2020. You call those Wii figures the writing on the wall, the time to move on. Actually, that's not what you call those Wii figures, because your post had no sincere interest in sales numbers to begin with. You started with the conclusion and tried to make the pieces fit. In the fiscal year ending March 2010, Wii software shipped 191.81m, so the following year declined by only ~20m.
Fiscal year ending March 2012: https://www.nintendo.co.jp/ir/pdf/2012/120426e.pdf
Despite Nintendo already being in the process of killing off the Wii (so no new big first party sellers throughout the fiscal year), there were still 102.37m units of software shipped. For comparison, the Wii U's lifetime software figure is 103.21m units.
Fire Emblem keeps being one of those things that people get wrong. The reason why it even got to the point that Fire Emblem may have had no future anymore is because Intelligent Systems made remakes of Fire Emblem 1 and Fire Emblem 3 on the DS, with the latter one not even being released outside of Japan. Of course that had to result in a sales decline. Fire Emblem: Awakening received the biggest marketing budget in FE history by far, and if Nintendo had pushed Radiant Dawn or preceding games like they did Awakening, then all of them would have comfortably been million sellers. Lastly, my scenario never specifically mentioned another Fire Emblem game on the Wii, so that attempted point of yours wasn't necessary in the first place.
Both the DS and Wii sold well enough to give Nintendo more time to prepare for the following generation. They had no rush, but they chose to do it and that led to a myriad of problems. The topic is still "did Nintendo kill off the Wii too early," not "how could Nintendo have handled the 3DS and Wii U better." Given what the Wii's software sales figures have been, it should be easy to tell that Nintendo killed off the Wii too early. And if Nintendo had released more and bigger games in calendar year 2011 and 2012, then obviously both the hardware and software totals for those years would have been higher.