Nintendo honestly doesn't want it to be a "second system" except to those who already decided they'd buy a PS3 or Xbox 360. Heck, it could be a "third system" or "fourth system" or even more if someone gets both other consoles and still plays an older one, or includes handhelds or PCs.
But let's not confuse "second" with "second-place". There's a distinct difference. Nintendo will use their place as "second" to come in first-place.
(It reminds me of an old Archie comic where Betty, Veronica, and three other girls were in a beauty pageant with their boyfriends as the judges. They all gave their own girlfriend two points, but for the remaining one point they had to give out, they didn't want to give it to any other girl who they suspected would be getting two points from their boyfriend. So they gave their second place choice to Big Ethel, who, because she got everyone's extra point, ended up winning.)
As has been discussed umpteen times before, the logic that Nintendo fans would buy a Wii and that other console fans would also consider getting a Wii would make the Wii have the largest userbase. I would think that that would entice many game developers to make games of all genres on the Wii - and even though they aren't high-def, uber-powerful, etc., the variety (and the controller and other things like lower development costs may allow for more variety than ever before) will eventually ensure that most kinds of so-called "hardcore" games - whether you define that as being "mature", or certain genres like RPGs or FPSes, will eventually be made. Based on possible sales and number of games, think it'll stay "second" for long?
If your definition of "hardcore" is strictly about power and that's what you want, get a powerful PC, with video and audio output to your high-def TV and surround-sound setup. I could actually do that, I simply have to move my PC into the living room, but I'll stick with being hardcore by catching all the Pokémon, thank you very much.