No, it's not time to let it drop. It's about moral leadership if nothing else - for all the reasons people have stated.
Apart from that, other people have been fined for doing what Cummings did. Moreover, the drive to Barnard Castle with impaired vision was an offence.
I expect people remember Chris Huhne and Vicky Pryce, who were prosecuted and served time in prison for perverting the course of justice. Huhne's parliamentary career was over. The original crime had been to claim the other had been driving, so that Huhne could escape penalty points for speeding. AFAIK no harm had been done and nobody was injured, but that's no defence. They broke the law and were punished. Once a society turns a blind eye to that kind of thing, it's on a slippery slope.
I think the current situation is worse because Cummings' behaviour is likely to affect that of others.