"The whole system of organ donation works purely on good will"
Exactly.
And good will breaks down when people know there are freeloaders abusing the system who will take but feel there is no reason to give.
Not wanting to donate to freeloaders who make the system weaker is different from not wanting to donate to Tories or to neo-Nazis or to alcoholics.
This is a medical situation in which some people will due because of a lack of organs to treat their condition.
People who directly contribute to that lack out of selfishness should be given a lower priority than people who are part of the reason we have organs to donate in the first place.
If people want to opt out of organ donation (not necessarily in a opt-out system) then that is up to them. No moral judgment should attach.
But people who want in on the benefits but not on the generosity should not be allowed to take organs from people who are fully on board with the whole thing.
Getting rid of the freeloader problem will increase the organs available, so there is a definite moral argument for doing it.
It's very different from bullshit about neo-nazis.
And bleating on about what characteristics "true generosity" should have is laughable.
Who cares?
What matters is that organs are available (with proper consent) and that freeloaders aren't able to bring down the system. As they currently appear to be doing.