To answer the thread's question, I think it will be Nintendo's Last Console. However, I think every company's current or upcoming console will be their last one. Our society is just moving away from this idea of the dedicated gaming hardware box separate from the PC or mobile device. So yeah, Nintendo will go handheld-only eventually because that's where our society is going.
As for the Wii U, I barely use mine. I think the most time I've put into it has either been the 10-12 hrs it took me to finish Disaster: Day of Crisis or the equivalent time in Lego City Undercover. I just have nothing but pure apathy for the device. I never play it, and I never want to play it unless I have an occasional craving for an obscure Wii game from my backlog. It's just not a device I associate with my gaming habits like I do my 3DS, Vita, or PS3. It has nothing I want to play on it, and it's a pain in the ass to use. Right now, my Wii U is out on loan to my best friend, who has also largely used it to play Wii & Wii Virtual Console games. It's not a device I'm eager to have back.
Do I think the Wii U is "doomed", though? Hard to say, and even harder to say if I actually care. Nintendo showed with the N64 that they could carry a platform almost entirely by themselves, but they made better software back then so that might not be a good indicator. For right now, all I can say is that this E3 will make or break the Wii U, so Nintendo had better have one HELL of a phenomenal lineup of games not previously revealed, or the Wii U probably will be doomed. Either way, it's not going to be my primary gaming device, so all I ask is that Nintendo give me a reason to care about my Wii U for those occasional games.
I do enjoy seeing Nintendo squirming, though. When Nintendo's bleeding and their back is up against the wall is when they're the most interesting, because that is when they are the most desperately creative. They were FAR too complacent and boring during the Wii years.