Even if the wii u had been a huge success as is the third party support would still not be great. The wii was hugely successful but most of the ps3/360 games couldn't be run on it due to its low power. The experiences were often poor for wii conversions and yet the price for the games was as high as 360 and PS3. Very poor value.
It would of been the same situation again this time around with wii u getting poor ports of the latest games that run well on pc, ps4 and xbone. Generally both wii and wii u were only worth buying for Nintendo games and a few other third party exclusives.
Lets not forget that Nintendo have the worst record for dealing with developers and publishers, often their fees are higher, the terms are worse and they generally milk third parties for as much money as possible. Sony in contrast has always been a much fairer player in this area with perhaps Microsoft in the middle.
I think there could have still been issues if Nintendo had bought out a console with a competitive spec that sold well in that some developers may have still considered not porting to wii u especially if the hardware wasn't x86 based and used a different gpu architecture than Radeon.
There is a nice camp now of triple x86 formats, PC, PS4 and xbone which is likely great for developers even if development software is different they know each format is relatively capable and what works on one will work on the other with minor adjustment in graphic detail, resolution etc. If wii u had actually been x86 based that probably would have helped it keep some support.
