Hevi-shot has the ballistic statistics that are better than lead and kills as or more effectively then lead, the only drawback is it's price!!! And we are only talking about a small number of hunters that want to hunt with older shotguns, I don't think the firearms industry is going to cater to that small of a market because there are plenty of shotguns, doubles included, that are built to handle steel and Hevi-shot.
All the best