The problem with more items and variations is that it slows down order turnaround, which costs customers. Its part of their current problem and why they have recently pruned some menu items. 5 years and more ago a trip to McDs was 3 min TOPS then they added new items, choices, menus ect. Now its 3 min standard and a lot of the time I just drive past, fast food should be fast and easy.
As for the onions thing, that employee was a tart, every McDs I have every been to takes "extra onions" as an option.