It is medically unethical to give a treatment that has no prospect of working. It is medically unethical to cause suffering with no purpose to that suffering. It is medically unethical to act outside the best interests of the patient, even if that patient wants the treatment that is not in their best interests.
Ventilation is invasive. It introduces a foreign body to the trachea (wind pipe) and then the mucous that builds up in the lungs has to be sucked up out of that tube. The mucous sits around in the tube and bugs can form. Nasty bugs. Bearing in mind that many people who have Coronavirus may well already have bugs in their mucous because they've contracted an opportunistic pneumonia from having the virus.
Ventilation is painful and distressing. Patients are given drugs to sedate them and drugs to stop the pain, but they need to be used very carefully, because those drugs themselves can cause bad side effects. So the drugs have to be carefully adjusted (titrated) according to the patient's consciousness levels and the way their body is responding to the drug. It isn't an exact science, so patients can go into and out of deeper and lighter levels of consciousness than is ideal. They can have dreams and memories from ventilated time, which will never, ever, make sense. If they survive, they may learn a 'rationale' for their experience, but it won't take it away.
If a medical team has come to the conclusion that a patient won't benefit from ICU care, it will be because the cost (in human terms) of having the treatment will outweigh the benefit to the patient.
All the time in normal times, patients are given options and given a prognosis. For example, a patient may be told that the only way to get them off of a ventilator is to have a tracheostomy. For one patient, that may be acceptable as the 'cost' of survival. For another patient, they may decide that a tracheostomy is not something they are willing to have and they opt for palliation (care that is aimed at symptom relief but not cure). It is a valid option that some patients choose to take.
We need to stop thinking of Intensive Care as some benign 'next step' in a treatment plan. It isn't. It's hugely invasive, lifesaving but life-altering treatment.