[ApK]

I strongly object to all these suggestions that have some variation of "only if it's popular" or "only if it's still making money."
They miss the point on both ends of the spectrum, the one being: why should the public domain only get stuff that's unpopular and no one's interested in any more, and the other being: Copyright includes the right to NOT let your work be published or sold or used in some ways you object to. Creators should not have to give up those rights sooner than others.
And aside from those extremes, in the middle ground, it's still unfair to deny a renewal or extension because it's not doing well in the market place. Many great works are ahead of their time, and only become gold-mine classics after many years. Don't deny creators that chance. One time period, one set of rules. Marketplace performance or presence should not be a factor.