HN and all other web sites are there for whatever purposes their users wish to use it for, within the constraints of whatever actions are implemented on a site.

This is one of the things that infuriates me about StackOverflow and its army of article closing moderators. A real community will change its practices and perceptions over time according to the needs of the community. If you have a subset of people who decide what a site is for, forever and ever without change, then it's not a community, it's a cast. Or it's a system of castes.

So if the community (through an influx of new users) decides that the purpose of HN is to spread cat pictures and memes, you'll be cool with that?

No, I'd probably leave. Unless the cats were really interesting.

Welcome to how so many social sites died.

It wouldn't be HN anymore at that point.