Everyone on these forums would more likely than not buy the game immediately, but that doesn't mean a stealth release is a good idea. To ensure solid sales, especially early on, you gotta build that hype machine, get people talking about the game, make sure it tops the charts.
I don't think I'll ever buy another Smash game. I owned Melee and bought Brawl, but never really got much out of them. I appreciate the fan service but the actual game part has never appealed to me. I don't enjoy Kirby platformers, I don't enjoy Smash, and I don't enjoy Uprising. I guess I just don't appreciate Sakurai as a designer.
You are correct though. Nintendo need to build the hype around Smash, not so much to ensure solid sales for the game itself, but rather to push sales of Wii U. If Smash and Mario Kart can't get it done, then nothing will.
I am kind of with you, I only had the first two, N64 and Melee and that was enough for me. Brawl was a rental and I got all I needed out of it that weekend. I think the fact these games keep getting made is part of what is wrong with Nintendo, it's all fan service lately but they stopped trying to get the stuff out we keep begging for.
Also I agree if this game can't move systems nothing can, but I lean towards nothing can at this point because Brawl didn't sell me on a Wii, that was built in, Melee didn't sell me on a Game Cube the N64 did that, so far nothing has sold me on Wii U and this isn't going to do the trick and I know I am not alone in this. Mario Kart is another one I lost interest in, Double Dash was the last console one I played and that was not my favorite. I know there is an audience for these games but I think many of them have moved on, so this could just be signs of Nintendo knowing they are done for.
As for a surprise release six months early goes, ask Sega how well that one works. If they did that then the comparisons to Nintendo being in Sega's shoes would never stop, so no a surprise out of the blue release would be a desperate move that only seals their fate.
The only way a Smash Bros. game will get me back is if they do another single player campaign like Melee had but cater it to each character, and again that only worked because there were too few full games on the Game Cube so it was for many characters the best we could get.