Further email from Canon:
"Graham,
Indeed, I know the 28-70 has been one of the most unfortunate cases as it was a popular choice before the release of the 24-70. I guess it is the case for models which have been superseded in some way. However there are lenses which have been on the market for 20 years plus which show no signs of being discontinued, never mind the extra 7-10 years parts retention period."
From that there may be some hope for some lens yet. But its fair to say for those who have a pre-IS or an newer version has been released of your lens (as in the 28-70) then its support days are limited