Of course we shouldn't have substandard care, palliative or not. Of course we should try not to let people suffer. It isn't like palliative care is at all affected by whether or not we have assisted suicide, as we can obviously have good palliative care without allowing assisted suicide at all (or the opposite, but we aren't that cruel yet).
But not matter how much we try to make folks comfortable, it simply doesn't always happen and not everyone wants to die slowly, even if they are comfortable in the meantime. Even the most comfortable care we can muster for the majority of folks doesn't result in a good quality of life, after all. (Of course, some folks are more comfortable than others just because of income and situation, which we can lessen but not get rid of entirely).
Assisted suicide shouldn't always come from the folks receiving palliative care, either. I'm fine with folks having plans in place to die if they have dementia, for example, or if their wish for a decade has been to kill themselves, despite getting are. Or heck, even if someone wants to die at x age, if they live that long. Well-planned assisted suicide should be available.
Many terminally ill people receive substandard palliative care, and want assisted suicide – it seems likely that at least some wouldn't want it any more (or might never have sought it in the first place) if their palliative care was better.
What are the ethics of saying to people "we aren't going to do anything to fix the substandard palliative care you are receiving, but we are happy to help you kill yourself"?