I’m not sure I understand your substantive question very well.

There already is a bridge from RSS feeds to ActivityPub: rss-parrot.net (there are plenty of sources I follow through that).

The clue of what ActivityPub is for is in the name: it is for publishing one’s activities. For example “I’ve written a new blog post”, “I’ve commented on someone else’s activity”, “I’ve upvoted someone else’s comment”.

RSS is really just a structured format to describe the content of a website in simpler terms. It doesn’t ever send any information to anyone, it doesn’t have any mechanism for anyone else to interact.

I used to follow news sites directly through an RSS reader. But I would need to set that up separately on each device, including after reinstalling, which I just can’t be bothered to do. I know there are things like Feedly, but not everyone likes proprietary services and software that much. I like the fact that on Mastodon nowadays, I can follow both microbloggers and RSS feeds.