Video games in general were almost exclusively a core experience about 3 generations back. There really wasn't an "expanded audience" to speak of.
I'd argue that it was the original Playstation that began the process of expanding the general audience. First console to sell over 100m units? The core video game market didn't expand that much over one generation it's pretty safe to say.
It's not that all those old 8 bit games are now "casual" games (many of those old 8 bit games are as hard core as ever in terms of hair pulling difficulty), they're simply viewed from a different perspective. Nostalgia for those who originally lived them when they were new, and maybe curiousity for those who didn't. They won't spend hours perfecting speed runs to post on YouTube, but they've probably played a lot of the old classics here and there.
Either way, it's still possible to have a game that appeals to both the expanded and core audiences. It's just a delicate balancing act that very few developers are able to do successfully aside from Nintendo.
The biggest problem with third party games on the Wii may simply just be an issue of publishers not wanting to use their A list development teams, or more specifically, spend AAA budgets on core games that the general Wii audience has shown time and time again that they just aren't interested in buying.
The core audience on the Wii (to include those who don't also own a PS3, or 360 or gaming PC) is almost definitely smaller than that on other platforms, so unless a developer can make a title with crossover appeal, they're not likely to see sales figures change dramatically in direct proportion to how much money, time and effort they spend.
Know the market for your product and choose appropriately where you want to sell it. The Halo franchise or GTA franchise simply wouldn't make the same numbers that they do on other platforms if they were "Wii exclusives."
