It means that in practice doing one thing means that you can do the other less well.
Let's assume that's true. So what? The argument under discussion was not whether the West should avoid focusing on killing people because it will undermine the West's ability to focus on cutting funding for ISIS. The issue under discussion is whether the West should avoid killing people because it will make other people angry.
Please don't try to change the subject without openly acknowledging that's what you are doing.
You don't do things like bombing or not bombing for a single reason. At the same time it's okay for an article in a mainstream venue to focus on a single reason because the medium doesn't allow for a deep analysis of all factors that matter.
Here's my op-ed that uses long-term orientation, probabilistic thinking, numeracy, consider the alternative, reaching our actual goals, avoiding intuitive emotional reactions and attention bias, and other rationality techniques to suggest more rational responses to the Paris attacks and the ISIS threat. It's published in the Sunday edition of The Plain Dealer, a major newspaper (16th in the US). This is part of my broader project, Intentional Insights, of conveying rational thinking, including about politics, to a broad audience to raise the sanity waterline.