With trademark, the test for a new name depends on whether the name can cause confusion with an existing trademark. When it's the same industry then it's pretty cut-and-dried in favor of the existing mark owner.

Reasonability of the name has nothing to do with whether or not Dropbox can use "Project Infinite". That's the point that Dropboxe makes. Dropbox can either fight that name (which really, they must or lose their own mark meaning anyone can use "Dropbox") and lose their Project Infinite name as well.