Putting on my tinfoil hat as I have not taken my normal pills- I think the dog is the only living creature in that house Robyn actually has some attachment to, and when the dog dies, she will be genuinely upset.
Just look at the sad photo she posted and the vocabulary she uses with Marley, calling him her "first baby." With most people, that would be odd but innocuous; however, Robyn's entire identity online is based on her own motherhood. Combine this with her vocabulary used in posts about Atlas. Typically, Robyn calls him words with negative connotations like "wild" and reprimands him frequently for doing normal baby shit, like touching Luna or her weird post about him starting to walk, and she often ignores him in order to play pretend that Luna the functional baby instead. So seeing Robyn call her dog her "first baby" makes me feel like it's some subconscious slip up; she doesn't actually see Luna as her firstborn child- in fact Luna's existence is less emotionally impactful to her than her pet dog. And Atlas is quite literally second thought.
As a comparison:
The family pet gets a real veterinarian, while Luna gets her fontanelles fondled by a man with no medical training. Atlas gets no medical care outside of a tongue circumcision and occasional woo.
The family pet gets held and reassured at the vet, while when Luna was still able to cry, Robyn just recorded it and laughed at her. Atlas is often treated as a nuisance by Robyn for the most basic things.
The family pet gets an extremely expensive and difficult to prepare raw meat diet, while Luna is forced to gag and choke on food she cannot swallow or taste due to her sponge brain. Atlas can actually eat, but is fed foods that are genuinely dangerous to children like raw honey for the "natural mommy" clout.
It paints a weird picture of a woman who loves the attention and elitism that having a "natural" homegrown spud gives her, but hates Luna's complete lack of interactivity. And while Atlas being normal and "all natural" helps with her residual regret from nearly guillotining a baby with her cervix, she obviously resents the lack of control that comes with having a real, functioning baby, and that bleeds into resentment to Atlas personally for "one upping" Luna by hitting his milestones, which itself is an uncomfortable break in Robyn's delusions that Luna is a sassy girl with tons of personality and wisdom who will totally recover one day if she puts enough coconut oil in her eyeballs.
And, well. The family pet actually responds if you say its name. The family pet does not make your own parents visibly uncomfortable with its swollen head, agonal gasps, and constant seizures. The family pet is not a living reminder of the biggest mistake of your life. The family pet existed before you made that mistake. The family pet still loves you unconditionally, and it can actually show it.
The family pet is dying, and soon all Robyn will be left with is Luna, Atlas, and Glenn, each of them a constant reminder that she chose to asphyxiate her infant child to spare her own pride: the living example of hubris.