The argument that you would be more likely to be "left to die" so your organs can be harvested is null, surely, because with the opt out system there will be a lot more organs to go around?
No, for a couple of reasons:
1) Removing the need for consent will not result in "a lot more organs to go round". It might result in a small increase. It might have no effect. It might result in a small decrease due to people objecting to the removal of the need for consent (which is presumably why it's been done on the quiet).
2) There are many ways in which an organ must be compatible with the recipient, so even if there were a lot more organs to go around (which there won't be) that still wouldn't remove the possibility of people deciding that the course of least harm is to declare a probably unsaveable patient dead in order to use their parts to save another, far more saveable, patient.
It's common for the extreme case to be described, in which a saveable patient is declared dead so that their organs can be used for someone who has enough money/power or who is more personally important to the surgeon, but it would usually be far more nuanced than that, far less clear in an ethical sense. Good medical ethics is based on the idea of doing net good and as little harm as possible. It's obviously not as simple as "do no harm" because that would rule out all surgery and most drugs. The point is net good and as little harm as possible.
So imagine this scenario:
Patient 1 will definitely suffer and die without a transplant and will very likely live well enough for decades after a transplant.
Patient 2 is greviously injured (shot, car crash, whatever), is almost certainly unsaveable and in the unlikely event that they could be saved they would have at best a short life of pain and suffering. But the organ that patient 1 needs is undamaged in patient 2 and is compatible with patient 1 and it would be possible to get the transplant done quickly enough. There's a team who could do it in the hospital right now, there's a suitable operating facility available right now and patient 1 can get to the hospital in 30 mins.
What is the course of net good and as little harm as possible?