It's hardly complicated!
OP is simply describing what is common throughout government in the UK. This is known as the Revolving Door.
Private Eye magazine wrote a special report on it some years back as, frankly, it is scandalous https://www.private-eye.co.uk/pictures/special_reports/revol... [PDF]
The revolving door is a completely different phenomenon, and it is a problem for a completely different reason. It's about politicians in positions of power over an industry later becoming business leaders or highly paid consultants in said industry. This is a huge problem because it works as a long-term bribe: instead of paying for the politician today and inviting inquiries and problems, you make an implicit promise to employ them when they get out of power as a future reward for preferential treatment today. This is an issue of corruption and excessive business influence of public policy.
In contrast, the issue with civil servants being let go and re-hired as contractors is a simple issue of inefficiency. The same person doing the same job has the same incentives, they're not being corrupted. However, our public resources are being spent inefficiently to hire the same person to do the same job for more money, with an added bonus of spending resources on the acquisition process itself.
Not to take away from your point, but the revolving door is also good because the experts should be the ones who make the rules. There are too many details about every niche that smart non-experts will get wrong and so we need experts.
Good luck reconciling these two conflicting points.
I don't think that's hard to reconcile - that only involves people in industry moving to government, which is far less problematic.
What happens after the job they were hired to do is complete?
I didn't say it is complicated. It is unusual for individuals not at the top of the ladder to set up their own companies and get exactly the same job twice. That's not what the "revolving door" is typically talking about.
Prompted and delivered!