A bit of a history lesson.
Only reason the NES was able to secure its place in the market and totally dominate the Sega Master System was because it forced 3rd party devs to sign a contract which disallowed them from porting their games onto other consoles. Thus, even though the Master System was more powerful than the NES, NES sold a lot more as it had all the games. And no, not 1st party games, as SEGA had plenty of great 1st party titles in the 3rd gen, but the 3rd party games.
For reference, Master System sold approx 13 million, to the NES's 61 million.
That's how important 3rd party devs are. They are the lifeblood of a console. And the moment Nintendo stopped having their support, was the moment Nintendo stopped being relevant.
The only thing I'm pointing out here is the hype machine surrounding a new console release, and how a lot of the PR talk and speculation is near identical to that of the WiiU, as well as a lot of what's happening as we approach the release of the console.
When the WiiU launched, tons of devs talked it up and down, saying it was easy to develop for, and that it was very powerful, and that "this time, Nintendo has got it, and they promise us tons of unique experiences on their console." Yet, a couple years later, we find that development for WiiU was a mess:
http://www.eurogamer.net/articles/digitalfoundry-2014-secret-developers-wii-u-the-inside-story
And very quickly we saw devs drop out, and even laugh at the idea of having their games on WiiU (Konami with MGR, Saint's Row 4, etc...). We are ALREADY seeing a similar trend with the Switch, they have a MASSIVE line up of games promised, over 80 to release over the course of the year. But, one look at the WiiU launch, and it TOO had a very large line up of games, 3rd party even. In fact, it was more impressive than the Switch, as IT had tons of games that were still relevant, whereas, upon closer inspection on the Switch's lineup, and you'll see tons of indie games, and games that stopped being relevant years ago, Skyrim, Rayman Legends, Cave Story, Binding of Isaac, a PS3 port of FIFA? It's not shaping up very well, and I notice a lot of artificial hype, and people trying to FORCE the success and acceptance of this machine.
However, like you said, the WiiU's failure was mostly due to marketing, not support or power. And you're right. But that's also where Nintendo has blundered the most with the Switch. They're marketing it as a home console, a successor to the WiiU and competitor to the PS4 and Xbone (whether they want to admit it or not), launching it MIDGEN while they other two are at their peak momentum. Not only that, but there's already a lot of confusion surrounding what the Switch is (is it a portable? is it a home console) in regard to the consumer. This is absolute market suicide. Who are you aiming this to? Outside of hard core Nintendo fans, who is gonna go out and buy a Switch when they have better, cheaper, and more powerful options already available? Not only that, but as a portable, the Switch is competing with the 3DS, which has a bigger library, and is much cheaper too. You've got an install base of 13 million WiiU owner, from which, only a fraction will hop over to Switch. And then?