You don't even need to be disabled for this to become a serious problem: just take a trip where you need to carry more than one large, heavy, or delicate item of luggage. Switching modes is easy[0] if you're able-bodied and able to travel light. If either or both of those things becomes untrue it's a different story.

[0] Note that switching modes always costs time, sometimes significant amounts of time. In some cases it's time you may not have or may not be able to easily find.