I have two quick comments in response to your question.
First, I think there's an issue with the timing of those decisions. Often they're made either right on the eve of an election or even in the opening days of an election when people are still trying to decide who gets access to which debates. The fact that political parties are directly involved in negotiating with the broadcasters means that you can rely more on.... Tactical considerations of whether it's in my interest to have you standing next to me are more prevalent than if you made the decision months out, before I knew where you would be in the polling versus where I am and so on.
I also think it's a mistake to make decisions based on a specific circumstance in which we have a party and the leader before us. One could imagine that you could come to a set of principles or a set of guidelines that would inform all such decisions around which you could agree. Are there thresholds you can imagine in terms of presence in the House at dissolution, or in terms of where they're running candidates and how many? Is there a threshold in terms of public opinion support six months out of an election that is now set in any event?
I think trying to find a way to say that if you meet the majority of these conditions you're kind of in is a more useful way than having to ask on the eve of an election whether so-and-so will get access to a specific debate, which is obviously problematic.