My theory, and I don't have any particular info to back this up, is that the decision to send a ride 101 or not has less to do with fielding guest complaints and more to do with keeping guests moving.
The New York Times article on the operational command center from a few years ago mentioned how they had managed to get the average number of attractions experienced by guests from 9 to 10. That seems small but the message is clear that getting people through as many attractions as possible is important. Send one ride, especially a high capacity omnimover like Mermaid, 101 for a while and it's going to have a ripple effect on the lines at other attractions. The result is that average of 10 attractions starts to dip. Guest complaints could usually be handled pretty easily by cast members without manager intervention so that's not necessarily a big concern for management. On the other hand, when the next level of management sees the park as a whole or a specific area was below their targets for the day, it's the manager who decided to shut the ride down who has to answer to that.
Again, this is all just a theory.