I don’t understand the question. When career people are promoted high enough so that they are the boss of GS-15s, they generally go into SES.
Another characteristic of the SES is that they report to the acknowledged political appointees.
So someone like the FBI Director is frequently from outside the agency while the Deputy Director is career and SES.
Sometimes federal law designates those second-in-command people be career. (Not sure about the FBI specifically).
I was going by what I read in the article:
The Senior Executive Service (SES) in 2016 had 8,156 members who were appointees. Obama appointed over 7,000 of them to these key positions. Most of these appointees do not arise from inside the respective agencies through a merit system and often are not qualified.
It is little known that there are many more SES appointees than any other type of political appointment. The “key bosses” in the federal government’s SES program are not only unqualified cronies, but they are also paid more than the highest government rate of G-15. They can even get bonuses, and each agency can set the salary of each SES member, which has no top range.