You raise a good point about development being far cheaper on the Wii, and that's an important point that that many people seem to forget. Remember, if you don't spend as much on development, you don't need as many sales to be successful. HD games on the other hand, many developers came public with the fact that they needed to break well into over a million to even make a return on their investment. While third party games on Wii may not have those frontloaded sales that the HD consoles enjoyed, many of them still sold respectively over time on Nintendo's console, especially when you consider the development costs were much lower.
Now, the question is, will it be similarly cheaper to develop on Wii U compared to Sony and MS's next consoles, or even their current ones? That remains to be seen. However, if that is the case, I think Nintendo will definitly have a leg up on the competition in terms of drawing developer support, especially in these hard economic times. The Wii U could very well be seen as the refuge console for third parties in hard times.
Everyone points to these mega franchises that have sold so well on the HD consoles like GTA, Metal Gear Solid, RE, Final Fantasy, etc, but people don't realize that these titles are extremely front loaded in sales. Meaning, the vast majority of people who buy these games buy them within the first month, or even the first week. This leads to deceptive numbers in the sales charts that make the games look like monsters. Then in the following weeks, they'll fall like a stone. On the other hand, look at games like Just Dance, or even more core games like Mad World, Monster Hunter, and RE4, that have kept creeping along in sales. Add the fact that most of these big buget HD games cost a fortune to produce, and suddenly the third party situation on the HD consoles doesn't look quite so rosey as you'd think. Especially if you are an indie dev with an extrememly low budget and minimal resources, Nintendo's much cheaper, much easier to develop for Wii U starts to look much more appealing to support.
In regards to the third party success of the Wii (or lack their of), third parties did better on Wii than many people think. On top of the huge success stories like Mario and Sonic, Guitar Hero, Carnival Games, Just Dance that have sold in the muili-millions, you also have the franchises that have done great numbers and no doubt turned a nice profit like RE4, Monster Hunter Tri, Sonic, EA Sports Active, Rock Band, Epic Mickey, Rabbids, Red Steel, Goldeneye, House of the Dead, Call of Duty, and countless others that have all of those have cracked a million sales according to vgchartz. Now, if you crack a million sales with the budget required for a Wii game, that's almost surely quite a nice return on your (small) investment. And let's not forget most of these games I mentioned didn't exactly require much effort, as they were ports, or games allocated to developers' "minor league" squads to make. I guess this was because the major developers prefered to create games in which they would lose money on for the HD consoles.
The bottom line is that third party games that deserved to sell (either by their widespread appeal or game quality), DID sell. The games that didn't deserve to, didn't. End of story. Any time people have used the third party doom and gloom argument for Wii, I've tried to get them to name even one Wii game they think deserved to sell but didn't. They can never do it, and the few times they could, they'll end up naming a game that is clear to have a niche appeal but that THEY liked, so I guess that means EVERYONE should like it. A vast majority of Wii third party games were either rushed ports, shoddy titles made by developers' B teams, or extrememly niche titles like Zack and Wiki that most Wii users haven't even heard of, let alone could get into. Now, while I may feel that a game like Little King's Story deserved greater sales, I also realize that an RTS/God game/adventure game like this doesn't exactly appeal to the mass market. Look at Okami in PS2. Sometimes games we think are great simply don't have that appeal to the masses, and thus don't sell. It just happens. That's the unpredictability of the market for you..
Considering the circumstances, (small budget, the lack of effort by most developers, competition from Nintendo games) Wii third party games did pretty respectable numbers, if not downright great.
