Let's say that someone other than Carolyn Colvin had been made the Acting Commissioner of Social Security and let's say it's a career employee in his or her prime working years. Let's also say that the Trump Administration decides a priori that Social Security will do just fine with a 20% reduction in staffing. That Acting Commissioner would face a dilemma. The person could forcefully resist internally in which case that person would probably be fired and their federal career would be at an end. That person could quit in protest in which case their federal career would also be at an end. That's tough on a person with a mortgage and kids in or near college.
At her age, Colvin can easily resign in protest. She has no reason to worry about her federal career. She can court firing or quit without concern. If she leaves, she can be very noisy about it. This gives her a certain power that others, younger than her, don't have.
By the way, if you're someone in line to succeed Colvin, what would you do as Acting Commissioner if the Trump Administration orders up something that you know will have disastrous effects? Would you have the courage to resign in protest? Would you be willing to preside over a disastrous situation? Is there some way of squirming out of the dilemma? These may not be abstract questions for a handful of people at Social Security.