I look at this two ways. One is understanding why Capcom didn't make this game on the Wii. They designed one game that could be made for both the Xbox 360 and PS3. They would've had to spend more time and money to make a version for the Wii. Also, I do think that hardcore games on the Wii can sell, there just isn't as much of a hardcore base on the Wii as either of the two HD systems. So while both console's copy of RE5 will sell at least 2 million if not more, a Wii version may have not done as much despite having a userbase almost that of both HD systems combined.
Now on the other hand, even if it did a million copies in sales, it would make a lot of profit. Also, it would be keeping the franchise familiar with those who might only own a Wii. For example, I only had a Wii until a few months ago. I would've been left out and maybe lost interest in the Resident Evil franchise. I don't see any way that this game wouldn't have been a pretty big success on the Wii. RE4:WE sold well over a million despite just being a port. RE:UC also did over a million even though it was just a on rails game for a full $50 while there were a couple cheaper options such as House of the Dead 2&3 and Ghost Squad. This shows that the RE franchise is very popular on Nintendo consoles. RE and RE0 did very well on GC. Add that to the success of the other games and there really shouldn't be any doubt that RE5 wouldn't be a big hit on the Wii.