True. Paul as writing to whomever he was writing to. However, that doesn't mean that (a) those writings are always exclusive to the intended audience; (b) we can't learn from his advice to them. This is why it's so important to read Scripture in full context, and to apply the Word accordingly.
You're being inconsistent in your hermeneutics. If the "we" is not applicable to anyone coming after Paul and the other Apostles, then (given the paucity of evidence for that interpretation), it should also be conclusive that none of the teachings of Paul (or Christ) for that matter, are applicable to anyone following them, too.