People can get better at writing songs with experience. (Or they can lose their freshness and sink into mechanically writing the same songs over again.) I don't think that songwriting as such improves - new fashions mean new types of songs, and changes in the world mean new subjects for songs.

It takes time to sort out which are the good songs in a new fashion and which aren't so good. By and large it's probably fair to say, for that reason, that an old song that is still sung probably has more likelihood of being a great song than a new song. That's where folk songs win out.