[tor-relays] How can we trust the guards?

Sorry for the naïve question, but we have a total of about 7000 relays, many of them residential and thus practically unused or very lightly used. So the actual number of relays that carry most of the traffic is rather small, and many of them are middle relays, leaving an even smaller number of guard relays. This means that an adversary with a rather modest budget can easily take over half the guards.

Whats’ the trust mechanism (if any) to ensure that the majority of guards are not hijacked by adversaries?