And how do they know that the chosen plan is not already optimal? How many reports do you think you'd get if every time someone encountered a "slow" query they reported it? And what fraction of those would be finally found to be caused by a bad optimizer as opposed to the user's fault like a missing index, outdated statistics, ... 1%? 0.1%? See stack overflow for a sample of that ratio. Be careful what you wish for.
If hints existed performance reports could be "default plan of x is slow, I know because when I use hints w,z then it's fast".
What should they do in the meantime while they're waiting for the pg project to acknowledge the problem, someone to propose a solution, and someone to implement a fix, just sit on their hands and twiddle their thumbs while their database server spews smoke? Hints are an escape hatch that empowers users to take over a when the optimizer veers off course. Not having any way to control the plan is like a tesla autopilot that doesn't allow the driver to take control over the car in an emergency.
Also, distrusting users to report performance issues is a weird attitude and gives a bad taste. Maybe it's true, or maybe you need to make it easier.