There is one thing I've never understood:

If one has enough award miles to get entire round trip for free,
why it's so popular to take all those miles and waste them on premium cabin
upgrade on existing trip?

Even if the on paper value is better for upgrade (500$ economy, 1,500$ Business class, so by upgrading you get $1,000, and entire new trip worth only $500).
For the casual savvy customer it's still clear that better have another trip for free.