Yeah, Nintendo never disappoints with backwards compatibility so I'm not worried despite having obtained the Wii U only 3 weeks ago or so.
Heck, my sister-in-law gave us her Wii a couple years ago, with almost no games (Rock Band Metal Trax, Guitar Hero 3, Mini Ninjas) and the few times we booted it up it was to play GameCube games.
On that note, as someone who considers themselves a "hardcore" gamer, I never bought myself the first Wii because I felt I would only buy Smash Bros Brawl, Zelda: Skyward Sword/Twilight Princess, and maybe 2 or 3 other games. The whole concept of motion controllers (and later the copycat products by Sony and Microsoft) felt gimmicky to me and beside an afternoon of casual fun not really trying to play Resident Evil 4 with a cousin, I saw little potential for it (as far as entertaining me, I should clarify).
Something similar happened to me when they first announced the Wii U. I thought "seriously? A tablet controller?"
And I think a lot of us can agree the Wii U took a very long time to get off the ground as far as available titles for it, especially Nintendo's main IPs. That translates into slow consumer adoption, but I think with the wave of games released in the last several months, adoption has increased significantly.
And now that I have it, I think the GamePad is absolutely brilliant! But even more so is the availability of the Wii U Pro controller (which I don't have) for "hardcore" gamers such as myself, as well being able to make the most of the Wii Remotes a lot of people already had from their Wii. It's just incredibly nice to have so many options to fit your style and budget.
I think a lot of people here are skeptical about the Wii U and the upcoming NX, and definitely strikes me as folks in these forums not having much love for the Wii U, but as someone who thought the first Wii was barely a step up from gaming on my phone, I'm excited to see Nintendo getting "back on track" so to speak, even if as a businessman I think their strategy was poor for achieving commercial success.
Let's also remember the Wii U came out way ahead of the PS4 and Xbox One and has been out since 2012. It turns 3 this year, and if they release NX in 2017 it'll be 5. Not much compared to last gen's 8-year rule (or so), but not far from the gap between the N64 and GameCube (1996 to 2001 = 5 years), especially if you factor in that lots of people keep saying this new generation is already outdated (and let's be honest, Nintendo has been behind in hardware power for a while now).