It isn't entirely secure from that issue. That is why Vote Decay was added to EOS Block Producer voting to mitigate that type of issue.
In theory if people aren't doing much then we can call it into question and people would unvote for those witnesses but more often than not that doesn't take place. A lot of people just stick in place and since they have powerful votes they just hang out there.
There could be the vote decay and a situation where they rotate out but SURPRISE..... none of the people in power like my ideas!!! Ultimately it won't get added to this blockchain.