I think it is common for non-frequent fliers to hold airlines to a higher standard than they can provide.

In many industries delays, cost overruns, etc occur at a certain frequency. Likewise when flying, delays and lost baggage also occur at a certain frequency. We pick and choose which businesses to interact with based on a lot of factors, not solely their performance in a single situation.

With airlines I see too many folks saying, "I'll never fly XXX again" because they missed an evening connection and had to overnight in an airport, etc. It happens to all airlines. However there seems to be a "feeling" amongst some fliers that this is an extreme situation, sometimes escalating into verbal abuse on powerless agents and social media platforms.

I don't see anything exceptional about UA delaying (assuming they deliver it) someone's baggage. If he gives it some thought he will also realize how out of place his anger is relative to most things in life, or into traveling in general. Sounds like a case of DYKWIA.