It avoids being fashionable and therefore never appears antiquated. Unlike fashionable design, it lasts many years — even in today’s throwaway society.
Long-lasting is a tricky word but still Rams’ statement is very clear on its meaning. It doesn’t mean an object needs to just last a long time itself, it means timeless.
Of-course that “good design is long-lasting” means to last long time, but not just the integrity of an object but its purpose, its appealing, its beauty.