Genie - they say HRT only protects while you are using it and that once you stop calcium loss quickly catches up, so you would need to be on it very long term indeed. Maybe justified if you have more than average tendency to osteoporosis, but there are other medications beside HRT that help maintain bone mass.
We can't escape aging; but laying up strong bones while we are young, a healthy diet and weight-bearing exercise will all help to lessen the degree of osteoporosis we suffer.

You ask "Do we know that plant estrogens will NOT have the same risks that animal estrogens do?" We don't know much about the effects of high-dose supplementation, though there is evidence against too high dosages of soya. But people have been eating beans and pulses, yams, and other vegetables in as dietary staples for centuries and generally the evidence is that a diet high in phytoestrogens in their unrefined form is healthy. It is also known from laboratory testing that these plant oestrogens are weaker in action than the oestradiol, the strongest of the human oestrogens. And there is statistical evidence that diets high in soya products are protective against breast cancer rather than the reverse.