Currently one can choose to accept or decline an appointment, or list it as
"tentative." Unfortunately, if one declines an appointment, it is removed
from the calendar. The suggestion is that one be able to optionally decline
an appointment without it being removed from the calendar.

Why would one want to decline without removing the appointment? To keep a
record of the invitation, or to remind one that one should follow up. Perhaps
the calendar is in flux and one may have to accept a meeting previously
declined, etc.

I find that many people (including me) seldom decline an appointment,
because of the side-effect of removing it. Instead, they reply with
"tentative", mentioning in the text that they will not be attending. I then
change the "show time as" value from "tentative" to "free" to help me
remember that I did that.

The problem with doing this is that the meeting organizer can't tell easily
determine the true number of people who have declined. Thus one should be
able to decline an appointment while still having control over whether it is
deleted from the calendar or not.