I will agree on one thing: yes, the author is milking this one plot device for all that it's worth, stretching it in order to exploit it as much as they can? Yes. So the question is, is it a good decision of the author, to milk/stretch this one plot point/plot device? Well, it's not like it's gratuitous (as this grandma issue was the very thing that kickstarted the plot, and may also continue to affect it, big time), and the decision of milking this one plot point, rather than the others, depends on how "deep" the author wants the story to turn (and not just on how "dramatic" they want it to turn) because the one topic they picked this time Is one of the more "universal" and "relatable" themes that have been available, so far, in this story (not every reader is a debtor, and not every reader works a professional sport, but EVERYONE fears cancer and loves grandmas...). The author clearly wanted to exploit the more "relatable" themes latent in the story, in order to avoid it coming off as "too convoluted" or as "too shallow".
Furthermore, believe it or not the story's been showing us the bare minimum of the ugly and *crude* images of old age and/or cancer: bathroom issues, lost hair falling around and needing to be picked up, not to mention x-rays, chemo... The art has barely filled the minimum to make sure that the character DOES look like an old terminal cancer patient..
i hope dan's grandma dies cuz shes really ugly and i dont like having to stare at her wrinkly face. she looks like shes been decomposing for 6 months. luv her and all but.... yeesh