Each message uses a new route and therefore the chance of selecting a compromised route increases with the number of sent messages. On the other hand if the Providers are not compromised then the rest of the route being compromised doesn't immediately leak who is sending and receiving message. Of course if Alice's Provider and Bob's Provider are compromised then it should not be necessary to compromise the mixes in their route because a long term statistical disclosure attack can instead be performed.
Each message uses a new route and therefore the chance of selecting a compromised route increases with the number of sent messages. On the other hand if the Providers are not compromised then the rest of the route being compromised doesn't immediately leak who is sending and receiving message. Of course if Alice's Provider and Bob's Provider are compromised then it should not be necessary to compromise the mixes in their route because a long term statistical disclosure attack can instead be performed.