So there are a lot of people like this:
But the thing is that sometimes this is just a sad attempt to try to get them to stay.
Even if you say "But it's your choice" afterwards.
(btw this is not targeted to "Bye, I'll miss you". I'll get to that soon)
The thing is that we shouldn't try to get them to stay. People come and go. Accept it. Move on.
My idea is to just notify that you know that.
Although another idea is to tell them your experience with them. Let them know how they've influenced the forum or your personal experience with them. Be honest, as usual.
Ok, about the "We'll miss you" thing. Yes, you will miss them, but eventually you will move on. But that's good because better will come.