You are right! No-one should be happy to see anyone leave the Anglican Church of Canada - whether it is one member of a congregation or ninety eight percent of a congregation. It is sad, and no-one wins in the situation.
However there are times when it simply has to be accepted that some have decided to leave, and the mission and ministry of the church in that place has to continue without them.
When this kind of thing happens it has to be remembered that while people leave and even whole congregations leave, the parish continues to have its identity. This is because a congregation is a group of people, with freedom to come and go as they please, but a parish is an integral part of a diocese. It can no more secede from a diocese than a municipality can secede from a province.
Further, while people may leave a parish, the bishop continues to have responsibility for the ongoing life of that parish. He can no more divest himself of that responsibility than the parish can decide to have a new bishop. I saw this situation in another diocese, where the continuing parish, after about eighty percent of the members left, along with their priest, continues to flourish to this day.
Of course members of a congregation have a deep-rooted love for their building, but they can never own it. It belongs to the parish, under the umbrella ownership of the Diocese. As a member of a congregation I do not own a "share" in the facilities. If I choose to leave, I have to leave it all behind, and the life of the parish continues.