NTA

A cat is a lifelong (life of the cat) commitment and it's not wrong of you to want to live with him until his death. This is a situation where I would usually say no asshole, just two incompatible people who should go their separate way, but here, I'm going to say she's the asshole for making you believe she would be willing to move in together despite the cat for so long, only to change her mind last minute.

Margo thinks I’m putting François above our relationship

Ultimately, you sort of are - there are 2 things you want that are incompatible and you are choosing living with your cat over living with her - but that doesn't make you the asshole.

she thinks this is some sort of ultimatum, but it’s not. I don’t want to break up with Margo, I definitely see myself spending the rest of my life with her, and after François passes I’m fine with not getting another cat,

It's not an ultimatum exactly but that decision is most likely signing the end of the relationship. It's not realistic to expect her to wait a decade to move forward with your relationship and start living together, when you guys agree that you want to and are ready now. But again, that doesn't make you the asshole.