I suppose one could always go off on a philosophical riff along the lines of:
"It's amazing how little any individual is really missed in a workplace, however long-serving and valued they are. In my working life I've seen many key workers with big personalities leaving a team while everybody wondered how on earth we would manage without them, and a few months later we could hardly remember what it had been like having them there. And when I've gone back to visit workplaces I'd left, I've realised that the gap I'd left behind had just closed up seamlessly behind me, too."
If one wanted to convey the truth - "no, we don't miss you" - without being rude or overtly dismissive, I think that would just about pass muster. There's no way to make that news not-hurtful to a person who fondly hoped to be told the opposite, but I don't think it could legitimately be held against one, and it lets them keep their dignity. Though TBH I don't think I'd bother, and would either bean dip or give a joke answer.