I think the reason he is losing so much High Level Workers is because his corporations are all upgrading at once and his education output is too low to do anything at all.
For example, an Air Transport Corp, while its quality and efficiency levels are increasing, so does its demands for High Level Workers. They need atleast 2.5 times, and sometimes 3x, the base number of High Level Workers.
The answer of this problem is simple. Build more universities to produce as much Engineers, High Tech Seniors, High Tech Executives and Executives. Also try to increase the education priorities. For me, my priority for Engineers is at 26, High Tech Senior at 15, High Tech Executive at 16, Executive at 16.
Now I am producing 14,500 Engineers, 3100 Seniors, 429 H. Executives and 617 Executives.
Just as Andy said, people dont disappear, they were simply recruited or employed by some of the corporations.
Also try to keep your population as young as possible. Older population means the country has a more older population. Once this old people reach 65, they will retire. Also try to keep the health index as high as your economy allow.
I think the problem here is simply mismanagement on part of the player.
Sincerely Yours,
The President of the Asian Empire in LU
The CEO of Xia State Enterprise in LU