You're right that if you refuse to accept any excuse for not performing [habit], the chain rule could be demotivating. But this is just a misapplication of the rule. The sensible reason to use the chain rule is to prevent a failure of volition. It's meant to address the problem where you say "I won't perform [habit] just this one day, because [reason]," and then before you know it, you've lost the will to ever [habit] again. If I ever decided not to journal, I do not think I would forgive myself. I forgive myself for the couple of incidents where I made no decision at all.

I think the last time I forgot to journal was... maybe a year and a quarter ago? I fell asleep before I intended to, and when I woke up, realized I had missed a night. I have never ever decided not to journal on a particular night, and this fact is enormously motivating. That is why the chain system is useful.

One interesting question is, how do I journal when it is extremely impractical? The answer is, I've done whatever was necessary to maintain the chain. Lacking a journal, I've used a pen and scrap paper. Lacking any writing utensils, I've used my phone. Lacking my phone, I've borrowed someone else's phone. The important part is that I made the sincere choice the perform the action, in however nonstandard a way, and this reminds me of my commitment. I've essentially inducted myself into my own religion.