Hmmm. I shot what I consider heavy bows (60#-70#) during my 30s, 40s 50s and into my 60s. Nearly 75 now, and I'm shooting 45#-50#, not because of any shoulder problems. Just not as strong as I once was. Never had and still don't have shoulder problems.
Heavy bows don't cause shoulder problems IMO, unless perhaps one shoots them improperly. A lot of folks injure their shoulders during a lifetime of living, and blame it on shooting heavy bows. Might be that the heavy bows just alert them to the fact that they already have bad shoulders.
Turkeyfoot: Did you place a bows wanted ad? Most folks don't list heavy bows for sale because there's very little market for them nowadays. Bet a lot of folks have a bow or two they would sell.