I believe you should be able to end it from the point of diagnosis if you wish, yes. Because it's your life, your body, and your autonomy.
I don't agree that it's a blanket statement of "that's a life not worth living" - it's an individual and personal statement of "this life is not worth living for me".
It's the same as abortion - some women are very happy to be pregnant, and some are not, and it is not usually argued that having an abortion means you must believe that having babies is a generally undesirable act, just that it is not something that would be positive in your own life at that particular time, for whatever personal reason it may be.
Where each person draws the line is different for them - some would want to attempt curative treatments at all costs, some would want palliative care and some would want to take an early opt out. All of those should be available to them, and they should be able to change their minds and switch between treatment options as well if they wish - because this is all part of having autonomy over our own bodies.
We do need to put sufficient safeguards in to minimise risks of people being coerced when it's not their own genuine desire, but I do think that is possible.
And it has already been proposed that medical staff will not be obliged to take part if they are ethically opposed, so anyone who chooses to participate in this kind of care would be someone who sees the value in it, as many people do.