I'll be honest, the only argument I accept against assisted dying is if the patient can be shown to be incapable of making informed decisions themself (i.e. dementia, degradation of mental acuity, etc). I think we try and keep our loved ones living in poor health too long, because we don't want to let go.
I saw it happen with my grandma on my mom's side. She was in her late 70s, and had lung cancer. She was not going to recover, and she knew it, so she decided not to seek treatment. My mom flew out to be with the rest of her family as they convinced her to go on chemo and radiation therapy, and she did, because she loved her children. But by accepting that, they removed her agency in choosing her method and time of death, and instead she hung on for maybe eight more months, lying in her recliner because it was the only place she could be comfortable enough to sleep, and slowly waiting in pain for the end as her breaths got shallower.
I look at that situation and I see my mother as being selfish. So you weren't ready for your mom to die - No one ever is. It's not like she's some dog you can keep limping along because you're too attached to see that you're keeping them alive for you, and not for them. She deserved the right to die, then, even though Canada didn't have it at the time. She deserved to be able to avoid those months of pain and waiting for the reaper to show up like the ghost in It Follows.