With regards to your example, you try to fix the gap between "consumption will increase" and "that will be a bad thing as a whole" by claiming little good use and much bad use. But I don't think that's the strongest way to bridge that gap.

Rather, I'd suggest that the good use has negligible positive utility - just another way to relax on a Friday night, when there are already plenty of ways to relax on a Friday night, so how much utility does adding another one really give you? - while bad use has significant negative utility (here I may take the chance to sketch the verbal image of a bright young doctor dropping out of university due to bad use). Then I can claim that even if good-use increases by a few orders of magnitude more than bad-use, the net result is nonetheless negative, because bad use is just that terrible; that the negative effects of a single bad-user outweigh the positive effects of a thousand good-users.

As to your main point - what to do when your best effort to fill the gap is thin and unconvincing - the simplest solution would appear to be to go back to the person proposing the position that you are critically commenting about (or someone else who shares his views on the subject), and simply asking. Or to go and look through his writings, and see whether or not he addresses precisely that point. Or to go to a friend (preferably also an intelligent debator) and asking for his best effort to fill the gap, in the hope that it will be a better effort.