They already do that, and that just results in constant pressure on the goal until you can finally punch one in, quite often.  Instead, with no offsides you'd have the luxury of keeping one or two guys deep on the opposition side to run out.  The defense could counter that by hanging back too.... and the end result is a more open field overall, which should increase the scoring, not decrease it.  i.e. if you can maximize 1on1 or 2on2 situations, that is more likely to result in more scoring than 5on5 situations.

Of course, this would have to be proven... but in pick-up soccer where there is no offsides called you usually see MUCH more scoring, so that's at least something.