I see your point marked one, I agree with you.
Its hard to argue that a really good 2d game wouldnt be better if it was done in 3d (with a 2d perspective) like Trine.
But you also have to take other things into consideration before you decide which path to choose.
With C2 you can complete a commercial project by yourself (in relatively short time frame). The assets are easy to handle and quick to produce. Progress is extremely quick.
Designing the same game in a 3d engine for commercial purposes, you will need a team (for a much longer period of time). The assets are time consuming to create and tough to handle. You will hit barriers and progress will be slow.
Im not trying to talk you out of it. If you have the time, skill and an iron will Im sure you will make a great game! Still though, I think the more logical approach would be to make your game in 2d and then approach investors to bring it to 3d for you. It will be the same game after all.