The flight is for someone who is 5 feet 3 inches. The more I think about it, the extra 3 inches, doesn't seem to be worth it especially since it is not Premium Economy, which would have been a totally different experience. I will continue to seek feedback.
Separately, if United doesn't sell all the Economy plus seats (whether newly ticketed, or existing tickets for which a seat upgrade is sought, will they let anyone choose the economy plus seats for free, 24 hours from the travel date, or would they actually let the seat remain empty as a wasted empty space?