It may not be such a pointless move. The nearest equivalent I can think of where similar moves have been taken to some surprising effect is plain packaging of cigarettes. One need only look at the millions of pounds tobacco companies spent on legal attempts to stop this initiative to deduce that it matters to such companies how shiny their boxes are; and likewise it matters to their customers.
I won't go too far down this line, but it's a useful analogy. Signs are that changing the packaging of cigarettes has been reasonably effective, albeit in a modest way. But it wasn't intended to be the only tool in dealing with a health crisis. I should imagine that the same is true of cereal packaging. Reducing the appeal of the unhealthiest cereals to consumers is likely to make a difference, just not a massive one -- and, clearly, more aggressive measures will be needed in future.