Company appearances count
At the company where I was a senior officer, all staff were required to dress conservatively, not chew gum or smoke in front of customers, etc. Because how we looked and behaved reflected very visibly on us and our company. Same for United and the other airlines. Instead of upgrading, I increasingly pay a first or business class fare. I periodically sit next to people who work for the airline or have a family and friends ticket. I tell them how much I appreciate the airline and the people who work there. But if my seatmate is in flip flops or other inappropriate attire, I think instead, what is this company coming to?
What the dress rules should be I leave to the airlines. They in turn would be smart to get input from their own employees. But in the process, please remember, people flying on friends & family passes do indeed symbolize your company, for better or worse.