"Separate but Equal" was SCOTUS doctrine for many years. So if "Separate but Equal" precedent can be struck down why can't' "RoeV. Wade"?

The pro abortion nuts are trying to argue that since the "Roe V. Wade" was SCOTUS precedent since 1973 it cannot/ should not be struck down.

Why can some precedent be struck down but not others?

Should precedent only be allowed if it promotes the agenda of Leftist Loonies?
"Separate but Equal" was SCOTUS doctrine for many years. So if "Separate but Equal" precedent can be struck down why can't' "RoeV. Wade"?
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