Scoot let your grandma down. You certainly should complain to the airline: it won't change your gran's introduction to Singapore, but it could help to improve the treatment of of Scoot's less mobile passengers in the future.

Might be an idea to send a copy of your complaint to the airport. I understand they take their reputation as a "number 1" airport quite seriously. http://www.feedback-changiairport.com/ might be a start.