It's also worth remembering that the main intended incentive here was for someone who was going to be flying on a YBM fare to choose United over a different carrier (because of status), not to choose a YBM fare when something cheaper or more reasonable was available.

In the long-gone days, refundable fares were often in top buckets only and policy required choosing such, and sometimes on expensive routes the cheapest 0-day published fare could be B or M. Instant Upgrades are a relic of a different era (they used to clear to JN even), and this will just be their final death.