At the time, I thought it was sort of silly. Of course we're not going to forget. It's not something you forget. I don't need a bumper sticker to remind me not to forget. I don't need a bumper sticker to declare that someone else didn't forget. I assume, rightly I hope, that everyone who was alive and over the age of, say, six on September 11, 2001 will not forget what happened on that day. It's permanently etched in our minds. We don't need to have a reminder not to forget.
What I wonder, though, is will we forget, someday? If I had a "12-7 We Will Never Forget" bumper sticker on the back of my car, some people would have no clue what the fuck it meant. Others would wonder why I care about a football score. Hell, putting on 12-7-41 probably would only further confuse people or possibly offend people who rationally have put the Japanese attack on Pearl Harbor behind them.
Time marches on, and more importantly we move on. We have moved on a little, I think. I hope. The rawness of that day has passed, in differing degrees, for each of us that will never forget that day. At some point, we needed to go back to our lives. That day is not at the forefront of our minds anymore.
I will never forget 9-11, and I don't need to put a bumper sticker out there to say that I will.