Perhaps your theory is airtight enough to do so; but I am curious what you would think if someone came to you and convinced you that they had tried to acquire a character trait in this way and failed.

I would say "really? you're not at all better at perseverence than when you started? Not even a tiny bit?" And maybe they wouldn't be, but in that case my theory would be wrong. It seems to me that it would be very hard not to change in a permanent way after following a new habit for a year.