[V_Terranova_Jr]

I'm sorry, but this is basically incorrect on every point other than the limited tenure. Source: I work in this sector.

PMs often come directly from industry. Once they do, there is a period over which they are not allowed to make direct decisions that affect their previous company, but the previous company is free to bid on that PM's programs and speak with that PM. When these conflict of interest situations arise, the office appoints a delegate PM to make decisions associated with that company.

There are absolutely varying qualities of PM, but the majority of them would be considered a domain expert or subject matter expert in some technical area. Standard practice is for a new DARPA PM to inherit existing programs that are already executing as the old PM finishes their tenure. When a PM leaves, the office tries to hand the executing programs to a new PM who knows something about that field (or is an expert). That doesn't always happen, for various reasons, but they do try to do it that way.

There is a large contractor staff that does a lot of heavy lifting on both technical and programmatic fronts. The PM is the decision-maker, but the contractor staff are integral. These contractor jobs can pay quite well for senior staff or technical experts. Maybe not FAMANG software developer salaries, but particularly for topics in the physical domain, the salaries can be significantly higher than the equivalent role in industry.