As a musician I will say that many of the songs from the old songbook had great structural harmonizations. Many radical and progressive key changes that made for interesting listening - just from a purely musical perspective. They really stood out against “Christendom”’s bland hymnals.
Unfortunately, with the new songbook and the whole “simplication” theme, they managed to erase all of those nice musical nuances in favor of bland sounding material.
For example, look at the slight musical change they made in “Make the Truth Your Own”. They took out the one line of radical key changes (that was awesome) and replaced it with a bland progression. No true musician would have favored such a change so I have no idea what the thinking was behind those types of change.