If you watch entertainment news you would never have known she was still alive. These old stars were cast aside and forgotten.
In some cases but lots of them simply decide to retire and be done with it. Others like Cloris Leachman and Betty White are still working. Some simply can't work due to health issues.
If it weren't for William Hartnell developing arteriosclerosis and having trouble learning his lines, we wouldn't have Dr Who today.