Indeed they did, but it doesn’t completely blow until Poe’s attack. Han and Chewie believe that the charges may cripple the weapon, otherwise they wouldn’t bother, so there’s an expectation that the charges could be the difference between the weapon firing and the weapon being unable to fire. The way it has been in the original movie and the edits is that Chewie fails to act immediately when Han dies. In the original this wasn’t a big deal since the weapon was 2 minutes from firing anyway, but here it matters. Unfortunately, placing Chewie’s attack before or simultaneous with the Starkiller firing ruins the feeling of despair at the plight of our heroes.

One way to mitigate this problem is to use the DominicCobb idea of having the weapon target the Resistance base directly after the Hosnian destruction. Then we understand that the only useful purpose of the charges was to allow the X-wings access to the Oscillator.