Why is it always something like... "the monorail made me late" instead of "I didn't get to the monorail early enough to give me time to have a safe margin for my FP"? You did have some control of your situation.
Everyone had at least an hour long window. If you chose to wait until the end of that window to ride it, then I think that is a risk that you took and it isn't anyone's fault but your own if you don't make it in time.
Whether or not you are able to use the debatable 15 minute grace period is not at the discretion of the CM standing there, it is a decision made from above that the front line CM has to follow. If the park is really busy or the ride is behind due to a breakdown earlier they will not honor the grace period. Don't blame the CM.