Yes, the first leg of the same roundtrip costs either about the same or way more than the whole roundtrip. The examples are from the cheapest fares on United or AA first. This is what a person who books online will see. I think the $8-10k price examples are relevant here, no need to compare the $20k ones.

Althouh I can see your point, for someone to maximize the 150k miles on United the need to be paying around $14k (x 11 = 154k rdms) in two one way tickets. At such high fares, two one ways could be somewhat similarly priced as a roundtrip. Still, I think it is still unlikely it is worth it, as you can probably just buy 75k miles for about $1500 at 2cpm.