I can't add a whole lot of insight that others haven't already said, OP, but as a twenty-something who watched her mother die right in front of her eyes just a few years ago, I feel like I can relate to you here. It is traumatizing watching someone you love die in an "unnatural" way.
My mom got a cut on her finger, didn't feel so hot the next day, went in to the ER, they sent her home. She felt even worse the next day, went back in to the ER, and was dead before the next morning. Meanwhile, I was 2 hours away working when my dad called me that second night from the ER just to let me know she was sick. That was the first I heard of it. I left my job immediately, drove the 2 hours to the hospital. Arrived as they were performing CPR on her in the trauma bay of the ER. She was already gone. It was devastating. In a true twist of irony, it was found out that she died of sepsis (bacterial infection of the blood). My profession? Clinical Microbiologist.
People who haven't been through something like that don't understand what it's like. To watch the life leave someone you care so much about. It is absolutely one of the worst things you can experience. My dad was with her before she coded and said that she looked and sounded so afraid and I just could not get over the fact that she died being afraid because she didn't understand what was happening to her. I used to have nightmares right after she passed about being back in that trauma bay while she was coding, just screaming out to her that she was dying of sepsis to try to get her to understand what was happening so she wouldn't be so afraid (she was a scientist by profession, also).
Point is - I understand how scary the thought of being alone (and afraid) as you're dying can be. And you don't really grasp the severity of it until you witness it or go through it yourself.
So on the one hand, unless he's also been through it, there's really no way your boyfriend can grasp the severity of the situation for you. From how you describe him, he sounds like someone who is not easily able to empathize with those he loves, either. This would make it especially hard for him to understand why it would be important to you for him to be there with you as you were passing away. So I can understand why he might be a little perplexed at why you are "making such a big deal" about this - especially since it is such a specific hypothetical situation.
However, I think what my bigger concern would be for the relationship in general is his lack of ability to empathize with you on these matters. My SO went through a LOT of medical issues before we met and he is not necessarily out of the woods forever. If he said to me, "[kellysouthpaw], if I ever get really sick again, I want you to do [x, y and z] for me/with me" I would not hesitate. Even if it was something that would not be pleasant for me (and, trust me, being there while he passed away would be so completely unpleasant, given what I've been through already), I would do it in a heartbeat for him. Because it would be important to him (and, honestly, for as unpleasant as it would be, it would also be important to me!)
So I would see this as, at the very least, a bright yellow flag. Definitely something worth following up with him on. Try to get through to him not just the passing away issue, but the deeper issue of empathy (and his seeming unwillingness to do something for you because it's important to you).