It might be gameable, but I think if there were a web of endorsements and you charted it out you could gather some really interesting information and maybe infer who is trying to cheat it based on the groups that form around it.

I don't think there are any bots pretending to be human that have large followings that aren't composed mostly of bots too. Bots would endorse each other and create their own circles on the chart because they wouldn't be able to get legitimate endorsements from real honest people who are endorsing each other. Any crossover from the two groups would reveal who is a bad actor making erroneous endorsements.

That's also why clients would only show you endorsements from people you follow or have endorsed yourself. It wouldn't just be about collecting as many as you can, there has to be a real connection to be meaningful.