Convincing parents to intervene

Hi vets -

Incoming second year vet student here. I'm struggling to convince my parents to do some bloodwork and end of life care for my childhood kitty.

16 yr old F/S DLH, has lost significant weight since I last saw her in January (BCS 4ish -> 2). Her appetite is low, she's far less mobile/energetic, is struggling to adequately groom herself, and shows some of the classic signs of cognitive decline/dysfunction.

On palpation (with my admittedly severely limited experience), I didn't notice any outstanding abnormalities or masses. When in motion, she definitely seems to be sore/arthritic, and she often gets her claws snagged on carpet even when normally walking.

She is an indoor/outdoor cat (despite my protest). She does still go outside, but for much shorter time periods.

She was on solensia monthly at my request for a few months, and that seemed to significantly help her and perk her up. My parents stopped keeping up on that because it was apparently too hard to remember.

I have even offered to take her back home with me so she can get care at the teaching hospital, and they wouldn't need to worry about it. I mostly just want bloodwork done to rule out the major old kitty things like cancer, diabetes, hyperthyroid, or kidney issues. And to paint a better picture of how long she has left/if symptoms can be mitigated.

They have said they're not really interested in diagnostics because she is old, and they are just waiting for her to die anyways. My dad accused me of wanting to take her "for the vet experience." I don't want to take her; it would be a significant hardship on both me and her. I'm kind of unable to understand their perspective; to me she needs this help because she is old, not in spite of it.

Anywho. I know people say your parents are your hardest clients, I just didn't expect it to be this bad. They just don't seem to be taking it seriously at all.

I have tried comparing it to human conditions ("she's just a cat, not a human"), offering to take her to the local vet myself on my next visit home ("we don't want you to have to add that to your plate"), and suggested humane euthanasia to prevent further suffering ("she's not that bad yet, she doesn't seem like she's in pain"). Does anyone have advice on how to convince them bloodwork is necessary, short of stealing her?