Owning the copyright to a work grants the exclusive right to profit off that work, but so does owning a house grant the exclusive right to profit off that house. House ownership is not a monopoly because there are other houses, and you can build a new one if there's sufficient demand to justify the cost. Similarly, copyright ownership is not a monopoly because there are other copyrighted works, and you can make a new one if there's sufficient demand to justify the cost.

The negative economic effects of monopolies stem from a lack of competition leading to insufficient supply, but there's no lack of competition or insufficient supply of copyrighted works. Even if copyright were indefinite like other property rights, it wouldn't stymie the supply of new works, but rather expand investment due to raising the future expected profit.