I don't see a problem with the current scheme. It just needs to work the way it's supposed to, 100% of the time.
Anyone who thinks the 500 mile certs will make them happier aren't recognizing the dynamics have changed, in my opinion. UA consistently offers cheap P fares and cheap buy ups. Although it would be fun to read the threads about how people have so many certs and can't ever use them.
The competition to use certs would be fierce now, with fewer seats. Not like 10 years ago.
There just aren't as many seats for CPU or instrument upgrades on most flights now. And CPU is a systematic way to fill those F seats on flights where people don't want to buy first or use an instrument, especially the shorter hops.
I'll ask the question. If the current CPU scheme worked exactly the way it was intended, all the time, what's so bad about it?