If what they say may in due course be used against them in a criminal court, then IMHO they must be warned. That’s a well-established principle of the legal systems of most (all?) developed countries.
With great respect, there is no such "well established principle" - at least not here in Aust or the UK.
The "rule" requiring a caution applies only to a "person in authority". That means a person who is perceived by the accused to be in a position to influence the course of any criminal proceedings against him/her.
That would rule out journalists.
Tell-all books/internet posts/emails would all be admissible, provided authorship could be established.
The failure by a person in authority to give a warning does not have the automatic consequence that the confession is not admissible in court (at least not in Aust/UK). Even where there is a specific legislative requirement that a particular form of words be used when administering a caution, it has been held that a failure to warn, or a departure from the prescribed form does not - of itself - make the confession inadmissible.
Those who struggle philosophically with
that should consider the position in the UK, where the right to silence has been abolished. The standard caution includes, "You do not have to say anything. But I warn you that it may harm your defence if you fail to mention when questioned something that you later rely on in court".