Regarding elevator door sensors, once we accept – as others have pointed out – that their purpose is to protect people, limiting their function can have serious consequences beyond the issue of melacha on Shabbos/Yom-tov.
Design engineers consider a variety of potential passengers, including the hearing impaired (audio buzzers), the elderly who may take longer to clear the doors, toddlers who escape their guardians and even the situation of someone fainting in the threshold.
The doors wouldn’t close based solely on, say, a timer even in concert with an audible and visual alert and risk injuring someone still in the threshold.