That's not necessarily true.
Perhaps you are preparing for something much worse than your own death but equally improbable. Alternatively, perhaps you already have your affairs in order and there isn't much valuable stuff you can do to prepare for your own death.
Naturally it's not /necessarily/ true - rules-of-thumb aren't always true, they just tend to be handy in a large portion of circumstances. Their main utility comes from having them cached and easily recalled, so that you don't have to waste a lot of time re-deriving them from scratch; this utility comes at the cost of not being applicable in every conceivable situation, but doesn't mean they're totally useless.
Does something like this seem to you to be a reasonable rule of thumb, for helping handle scope insensitivity to low probabilities?
There's a roughly 30 to 35 out of a million chance that you will die on any given day; and so if I'm dealing with a probability of one in a million, then I 'should' spend 30 times as much time preparing for my imminent death within the next 24 hours as I do playing with the one-in-a-million shot. If it's not worth spending 30 seconds preparing for dying within the next day, then I should spend less than one second dealing with that one-in-a-million shot.
Relatedly, can you think of a way to improve it, such as to make it more memorable? Are there any pre-existing references - not just to micromorts, but to comparing them to other probabilities - which I've missed?