If you could be a God...

If someone is comatose, then the doctors may seek consent from the family to pull the plug on the patient. That's a case of having to decide one way or another for someone who lacks capacity to decide for themselves. If someone requests to die and assistance in doing that, then their consent can and should be taken at face value. 

In the case of a foetus, it is only by the actions of the aggressor that they are forced into a highly risky environment and are saddled with debts that can only be fully paid off upon death. In that scenario, the foetus isn't morally relevant (which is agreed by proponents of the right to abortion) and don't have anything invested in their own continued existence. Therefore, the morally correct thing to do is to deny the aggressor the right to impose risk and harm upon someone who could not consent. It can't be against the values of the foetus to do so, or against anything that they already have invested in their own future, and can only save them from future harm. And you're in favour of aggressive state intervention to deny people the choice to decide, and if they're never 'able' (by your standard), then they're going to have to suffer until the limit has been reached in terms of how long medical technology can forcibly prolong their life. The only option that you have for those who, by your definition, are not 'mentally competent' to decide to die, or complete suicide effectively, is to hope that eventually a treatment will come along so that they can graduate from an existence of relentless torture to the type of tolerably mundane existences that most people have. Absolutely no guarantee of being able to grant them their relief of course, but life is so infinitely valuable that they'll just have to accept the fact that their constant suffering is collateral damage that people are willing to impose on unfortunate others.