Yeah, the keyword here being additional construction. The OP wasn't talking about extending a program. He was adding strict for the sake of adding strict.

New functionality may need to be added to the existing program

Yes, but why spend time now for something that may not happen? It's not that if a program remains stable for another year, that you have to hit the keyboard harder if you are going to add strict in a year, when you do modify the program.

strict can help in that department but you would need it for the entire program.

Sillyness. use strict is lexically scoped.

Everyone would be much better off if coders stopped writting code that "works" and instead started writting code that works well, scales, and is easy to maintain.

Totally beside the point. The program has already been written. Besides, just slapping a 'use strict' on the program isn't magical wand. Writing easy to maintain programs that scale well takes a lot more than slapping 'use strict' on the program. In fact, there isn't much relation between them.

Abigail