Regarding your points:
"Threat Actors compromising an account..." You're spot on. A vouch-based system inevitably puts a huge target on high-reputation accounts. They become high-value assets for account takeovers.
"Threat actors creating fake chains of trust..." This is already prevalent in the crypto landscape... we saw similar dynamics play out recently with OpenClaw. If there is a metric for trust, it will be gamed.
From my experience, you cannot successfully layer a centralized reputation system over a decentralized (open contribution) ecosystem. The reputation mechanism itself needs to be decentralized, evolving, and heuristics-based rather than static.
I actually proposed a similar heuristic approach (on a smaller scale) for the expressjs repo a few months back when they were the first to get hit by mass low-quality PRs: https://gist.github.com/freakynit/c351872e4e8f2d73e3f21c4678... (sorry, couldn;t link to original comment due to some github UI issue.. was not showing me the link)