> hey often allow you to waive protections ... if both parties agree to it.
This is not really generalizable. Arbitration clauses for example waive one method of recourse in favor of another method of recourse, both already accepted by the courts. I expect you couldn't replace that with trial by combat and expect it to hold in court, but you might be able to argue that a different resolution process with historical precedent would hold - at least you could test it in court.
I'm not particularly knowledgeable on the topic, but there are all sorts of religious courts (at least in the US). Generally they coexist with the secular legal system.
Yes that's true, but again it's a case where (a) both parties agreed to (b) a recognized form of resolution, in this case mediation - you can do this with a secular mediator also. In general they don't "coexist" so much as they are subsumed within, as allowable forms of resolution, within prescribed bounds.
Important to note that the courts already recognize and/or have precedence for this, and also there are limits to what you can shift here. Finally if the resolution process is not acceptable to one party there are ways to bump it back to the courts.
None of this applies to "smart contracts", which aren't contracts in the legals sense anyway. There is no (yet?) agreement by the courts that this is a valid resolution method, and any issues of contractual law brought before a civil court will be resolved by legal principles, not source code. I suspect that the best you could hope for in the current setting is that the source code + associated communications speaks to intent.
There's no reason to believe that it is impossible that US courts and/or legislation would at some point give some legal status to a smart contract beyond above, but that's not the case today as far I know.
This is not really generalizable. Arbitration clauses for example waive one method of recourse in favor of another method of recourse, both already accepted by the courts. I expect you couldn't replace that with trial by combat and expect it to hold in court, but you might be able to argue that a different resolution process with historical precedent would hold - at least you could test it in court.