A few flaws with your counterargument: your point on team killing already being illegal is counter intuitive to your argument as if an admin is on, you don't have to bother with rewardening. Sure, someone could be reported, but that isn't an instant fix and there is no way to enforce them not becoming warden after team killing. It doesn't solve the main issue of you can't control this rule without an admin online, meaning it is really only problematic. It isn't about someone breaking the rule against team killing, but this exception becomes a reason to break it and nullifies ramifications for a mistake.
My point is that if somebody teamkills the warden, by accident or on purpose, they have
already broken the server rules, and rewardening directly after doing so would only further that. To exploit this rule, it
requires you break a different rule first.
If an admin isn't on, there is no instant fix to somebody breaking any rule, but reporting them is so that they are prevented from doing it in the future. You cannot make an argument saying that a rule should not be added because it cannot be enforced without an admin online, because that would mean that every rule is void until one gets on. You can't enforce rules like "no ghosting" without an admin on, but it still happens and is a rule. The way to enforce any rule without an admin online is to tell them it's against the rules, and/or to report
While it isn't often that groups of friends join, it isn't uncommon either. Sure, they probably will mfk, but that doesn't mean giving them another easy to break rule will solve that issue.
Even if two or more rule-breaking friends joined, It's highly doubtful that they will want to exploit, or even know about it's existance. In the specific case where one person does kill the warden and the friend rewardens, the person killing the warden would still likely get reported, and the new warden would either break more rules (especially with my addition), or continue the round normally. As I said before, a different rule has to be broken in order to exploit this one, meaning the incentive to kill the warden is nullified.
However, the biggest problem is your addition of not being able to make new rules. That only works in the context of hg, and nothing else. It doesn't work for normal day with ff (though that often turns in pseudo hg), pokemon, or warday with ff. It actively hampers those rounds from continuing on normally if the warden can not give new orders making it even more difficult to implement and harder to enforce. I still disagree with the suggestion and if it is implemented, I see the additional rule as not being feasible to work nor enforce.
In making this suggestion, I was mainly only thinking about hg and arena, (and possibly meatgrinder,) because they are defined in the server rules or by map design. It could easily apply to pokemon with only change needed being that the warden can reassign trainers if one is to die, as the only other exception to the "no new rules" addition. What I do agree with you on however, is not applying this rule to
custom lrs that involve ff, since they have the possibility to require new rules.
My suggestion was only meant to be applied to the pre-defined minigames, and not things like cellwars or other custom ff lrs. That may be a discussion to have in the future, but adding this as it applies to the minigames I said before, will help rounds be continued, and run smoother despite issues with teamkillers.