I doubt I will ever buy another Nintendo product as an adult male in his late twenties who enjoys mature games and content.

The last Nintendo product I bought was the Gamecube because of RE exclusivity.

It's not as if Nintendo are releasing bad games for it, in fact the exclusives are bloody fantastic. If great games cant move the system, then what hope do you have? The Wii U is not a bad system at all, its games are among the best we've seen this gen, but sadly lack of 3rd party and poor perception from a lot of ignorant gamers have contributed to the poor sales.

This is true. The seeds were planted during the SNES days but, the SNES won WW with ease so, I don't know if it's accurate to say the ACTUAL slide started there. I mean, DKC was the biggest game on the market in 1994 and that was a Nintendo exclusive.

Nintendo was able to effectively "answer" Sega. While they did get caught flat footed in the mid part of the generation, surprised by Sega's edgier marketing and abrasiveness, they did fight back uncensored Mortal Kombat 2, getting DKC of course, but also Killer Instinct and more edgy marketing of their own to turn the tide. 

The slide really begins in earnest with the N64 yes, mostly because it started to lack a lot of third party games because of the really awful CD-ROM decision. 

I don't think so, the Gamecube really damaged the Nintendo brand heavily, but then it much more than recovered with the Wii. Next generation will be a whole new playing field.

I don't really think so..

Nintedo is known for thinking outside the box and being innovative will not always lead to success.

I'd say the Wii heavily damaged Nintendo's image, more so than the Wii U

In bizarro world, maybe. The Wii was the first positively received Nintendo console since the SNES.

The Wii had kind of like a twin cycle, a huge upswing, and then a negative downswing. 

It also had that fad/craze cycle attached to it, where it was very popular at one point, but not as many people really look back at it in a hugely positive way. 

Obviously casuals moved on, so the ones left to critique it are core gamers and they likely weren't all that thrilled with all the Wii era. 

Nah, the Wii was very well received right up until the launch period of the Wii U. The downswing was largely directed on Wii U, which was comparatively unsexy in all manners.

Had Nintendo gone with a sexier console rather than the Wii U, then it would have sold extremely well. Although if the Wii has taught us anything, it's that anything can change with the launch of a new console. It's the same with any electronics line. Nintendo also has an advantage over the others in terms of their extremely well known franchises.