Earlier quoted context omitted.
Literally nothing in the legal system is permanent for perpetuity. Even the constitution can be amended in any way. All laws, treaties, etc. can be undone with enough effort.
Not quite. It is, in theory, possible to pass an amendment to the constitution, that would prohibit further amendments to parts of it. Quite a few countries have such "immutable clauses" in their constitutions.
Those countries can just make new constitutions. There is literally nothing stopping them.