For any piece of music, there comes an age when it is either forgotten or becomes a "monument", similar to buildings ("Let's demolish it quickly before it gets protected as cultural heritage"). Having reached that age and achieved some impact, it becomes an object of "history of music" where criticising its quality (other than by folk process) is a no-no.

My feeling is that 30 years is a suitable age for all genres, reduced to 20 for pop music. However, some songs become "classics" almost immediately when first published, without process.