IIRC from my 25 years of riding 19/20, there usually has been one or more employee standing at a vestibule to ask for locating the correct car. I have little tolerance for people who wander aimlessly up and down a platform assuming someone will magically appear to lead them by the hand.
This is fine if there is an employee standing at the vestibule and if the passenger can't read.
We are talking Denver here. The OP didn't mention coach or sleeper; but it doesn't matter. Of course sleepers have the car # and room #. Coach and Sleeper passengers now line up on the platform near the station with luggage in all types of weather to have their tickets scanned. Coach passengers are given a boarding pass with the car number. I definitely don't think it is unreasonable to expect some physical indication that matches the car number assigned. If it is considered bad form or impossible to have correct car numbers on the car, they should be duct taped over; there should be a movable sign on the platform indicating the car. Even European stations have Wagenstandanzeiger's that shows the position of each car.
Denver is an odd situation in that the paranoia factor has disappeared with the station reconfiguration. Before, when the train arrived, the conductors would get behind two desks (coach & sleeper), punch tickets, give coach passengers a boarding pass. At some point they would open the ramp to the platform from the tunnel. Before that time a rent-a-cop would keep everyone off the platform. Even if you arrived via Light Rail at the other end of the tunnel and wanted to rest with your luggage on the bench in the tunnel near the ramp to the train while your spouse handled the ticket stuff, the cop would try to move you.
Now, they don't even announce when the train has arrived or is boarding. You have to watch the small, low situated electronic board closely to know when to go outside. I say closely because the estimated arrival time can jump around, and then unexpectedly it's here.
The sign board will tell you the track number; but there are no signs indicating where it is. Experienced riders know to detour left, and make a U-turn around the end of the platform. Then it is easy enough to see the queue and train. But if you take the escalator or elevator down to the tunnel, there is no sign telling you which elevator to take to get back up to the CZ.
Once you get your boarding pass, it is still quite a walk to the train, presumably because the engine has to be spotted at the only place the fuel truck can sit.
In summary, it's almost like Amtrak doesn't exist at Denver Union Station; and I can see why non-AU types expect a little help.