The law and the contract you agreed to when you bought the ticket are what controls here, not what you think is "reasonable" or in "good faith" at a particular moment in time.
Here is United's Contract of Carriage document - in particular see rule 25. Here is the Federal Code of Regulations section regulating overselling. Notice the last sentence of 250.3(a) in the CFR (aka FAR) which governs the policies airlines must make regarding overselling which states that:
Such rules and criteria shall not make, give, or cause any undue or unreasonable preference or advantage to any particular person or subject any particular person to any unjust or unreasonable prejudice or disadvantage in any respect whatsoever.
This makes it difficult for an airline to consider special cases (such as true, or untrue, claims that "I have to see patients tomorrow") as if they do so and allow Passenger A who was initially selected for removal to remain on the flight, they now have to now remove Passenger B who would have otherwise remained on the flight -- opening the airline up to a lawsuit from Passenger B. Thus, the last resort of random selection among the lowest priority passengers.
If you fly United, you agree to all the rules above. If you don't like them, don't fly United. If it's really important that you get somewhere, pick an earlier flight (and certainly not the last one of the day) or have a backup plan (another airline, another flight, a private charter on call, etc...).
I don't like overbooking but it reduces the cost of airline tickets. Perhaps a new set of fare classes could be added which would guarantee a seat. These tickets would probably still have restrictions but would be just like what you seem to want. They might cost quite a bit more, they might be completely non-refundable/transferable unless the flight departs more than two hours late, they might require checking in 30 minutes before scheduled departure (at which time they would become standby tickets but still be non-refundable even if you don't get a seat because all seats are filled by other standby passengers).
Perhaps you should start an airline that differentiates itself by not overbooking at all if you feel strongly about it (the CFR doesn't seem to require overbooking, just allow it). Maybe it will be so popular you will get rich and put the rest of the airlines out of business (although I doubt it -- most passengers are very price sensitive).
The odds of being involuntarily booted from a flight are tiny compared to all the other reasons you might not get where you expect at about the time you expect. Other risks such as mechanical failures, crew shortages, computer failures, weather holds (even not at your source or destination -- but the plane planned for the flight is stuck on the ground at a third airport) are far greater risks so it is reasonable to assume that all passengers are prepared for failing to get where they expect at about the time they expect.
BTW, if someone fears being denied boarding due to overbooking, there are several ways to dramatically reduce the odds of being selected. One is to buy a higher class (business or first) class seat. Another is to check luggage (the airline doesn't want to delay your flight by having to find and retrieve your luggage from the plane). Another is to join the airline's frequent flyer club. Another is to become disabled -- perhaps chop off a leg or gouge out your eyes (but I really don't recommend this last strategy).