I don't follow how we go from "they are bad product managers and customers" to... therefor time estimates are bad. I do not think it is unreasonable for our primary stakeholders to ultimately care about time. I also do not think it is unreasonable to give error bars in estimates like "this project is uncertain, therefor estimates will be variable".
The bad managers are those who ask you for an estimate and then take it as a commitment.
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"How much time will this take?"
"Not sure; approximately 30 minutes, but could also be 20 or 40 minutes. If we are very lucky then 10 minutes, but if we are very unlucky, maybe an hour or more."
"Spare me the details, I just need one number for the report."
"Uhm, 40 minutes?"
"You just said that it would be approximately 30 minutes, didn't you?"
"Yeah, but I wanted to add some safety margin..."
"If we keep adding large safety margins to everything, then the project will take forever. As you said, some tasks are completed faster, some tasks are completed slower, on average it will cancel out. I need your best estimate."
"Uh, okay, then 30 minutes. On average."
...the next week...
"So, you guys told me this would take 30 minutes, but it actually took 35. I think we need to have a serious talk about your performance."
