We've all heard the good old wise saying: "If it ain't broken, don't fix it!"
My opinion is that this new policy is not "fixing" anything, but is actually breaking a system that has been working very well until they try to "fix" something that is not broken.
I can quickly think of a few consequences of this new policy:
1. This policy will encourage some passengers to drink more than they should before they board.
2. It will also encourage some passengers to smuggle and consume their own alcohol.
3. It will create and intensify unnecessary conflicts between passengers and the FA's -- It is perfectly fine (and is probably the law) to stop serving alcohol to someone who is intoxicated. But why would you not let a perfectly well-behaving passenger purchase a second beer so that they can relax and enjoy the flight?
Last but not the least:
4. This will definitely motivate elites like myself who mostly fly long flights to seek status match or, as many have posted on this forum, just forget about status at all, period! Approximately 80% of my flights are approximately 4 - 6 hours long and my international flights are 12 - 17 hours long!
Ironic that I just made platinum for the first time and UA is encouraging me to fly another airline!