It would be nice if they said goodbye, I mean if you die or go into a coma, you can't say goodbye. But as long as you are able to type, it takes two minutes to say goodbye. Nothing wrong with leaving, but if you are a long time poster, it'll be nice if you just say "I'm outta here" and that's it.
That is what layinwood, kegray1, BleedDodgerBlue and some others did, they just said "see ya" and left.
I am sure some on the list above have died and in that case nothing can be done, but I mean if you are a poster that posted for decades, it would be courteous just to spend one minute to say goodbye and then leave.