I’m mostly imagining what my reaction would be if one of my friends described a relationship by saying that if it isn’t fun anymore they’re done, but they’re also thinking of moving in together. Major red flags.

I think that mischaracterises what is going on here. The characters in the comic aren't describing a relationship to a third party. They are talking openly and honestly about their feelings to each other.

You're talking about relationships not lasting, and being unhealthy, etc. - but there are multiple people in this thread who have publicly said that this comic reflects their own long-term ongoing relationship. I think it's important to understand that different people communicate their feelings differently. To you maybe telling to your partner that you can't promise to love them forever a red flag; - whereas to me, telling someone that you will love them forever unconditionally is a red-flag, because life isn't really like that. I don't want platitudes or empty promises.

I agree that it's bit sketchy to say to your partner "if it isn't fun, them I'm off"; ('fun', I think, is a bad word to use.) But on the other hand, the two characters in the comic agreed at the start that they weren't even looking for a long term relationship. So they are on common ground. They aren't just pulling out a bombshell from nowhere; but rather they are acknowledging their current and changing feelings. So in their relationship it can make sense to say something like that.