That's a great point! One of the papers did discuss this (not with those exact words, but still). I certainly see it in myself.

At the same time, it's possible that people with high self-control might also bounce back faster after losing their good routines (e.g., when on a work trip and staying in a hotel or going on vacation). So even if they are fragile for a bit, they also adopt new routines faster. But this is just speculation based on the research I covered in my text. :-)