Let's think about adding netplay to a game for a while, all of you competent game programmers who are suggesting it's no problem to add it.
There's 2 ways I can see the VC being implemented: built-in emulator or do-it-yourself code. If DIY code is used, the company could opt to do a port or simply code an emulator and run the original code through that.
Built-in Emulation
This could be the toughest scenario for a developer to work with. In this scenario, the Wii loads a hardware emulator and then loads the game image and there you go. The problem with this is the developer can only work with what Nintendo gives them and has no freedom to add netplay to the game unless Nintendo provides it as part of their package.
DIY Emulation
This method forces the developer to create their own emulator and run the game image through the emulator. If the developer can make the emulator support netplay, the game may be able to have netplay implemented. I'm assuming what the X-Band modem did was transmit controller input over the modem, which was then sent to the cartridge, so the ROM didn't know it wasn't fooling with 2 controllers hooked to the same system. There's some synchronization of game issues I can see happening there but nothing is stopping the developer from letting the emulator handle that.
A nice bonus here is once the developer makes a solid emulator, they can use it for more than one ROM so the effort is only needed once.
Full Port
This gives the developer full freedom but requires the most effort. In this case the developer completely redoes the game and adds netplay as part of it. I honestly don't think many companies would go to this much effort unless they expected very high-volume sales out of it.
However, when you take all these things into account, not all games would be netplay friendly and people would still complain about the results. Split-screen games (such as Goldeneye) would still play split-screen, since all you are doing is sending controller input signals to the ROM. Reworking these games to support full-screen multiplay would involve some creativity to implement at the very least. Even worse, modern online multiply benefits from over a decade of refinement. Lag is almost always a problem, and most modern games account for lag in some way or another. These older games would either have to assume no lag exists or have code added to compensate for it, which increases development effort and decreases the profitability of the venture.
Writing games isn't easy; there's a reason why lately it takes years to develop a game. As the industry pushes itself to set the bar for production value higher and higher, it is also increasing the overhead for producing a game. I really don't think many companies are going to divert attention from their multi-million-dollar projects so they can make a quick $1-2 million off of making an online Goldeneye. I know it sounds tempting to you guys, but you have no idea how big a development project gets and I think most game developers are pretty much fully occupied as it is.
*edit* Oh and yeah Battletoads was definitely a rip-off of TMNT but I think the phrase "rip-off" isn't exactly appropriate because it has a bad connotation. Allow me to illustrate.
Voltron was an anime series about a giant shape-changing robot piloted by people. The same company procduced Mighty Morphin Power Rangers, a show about people piloting giant shape-changing robots. Both shows were successful, so MMPR is more like a derivative work of Voltron than a rip-off.
Tattooed Teenage Alien Fighters from Beverly Hills was a show about some people who controled giant beings that could transform into another being. This show was low-budget and not very popular. It is a rip-off.
So basically Battletoads isn't really a rip-off because they did a good job, IMO.
