I'm excited to see the Threads / #activitypub experiment play out.

Threads is going to have a much larger user base than Mastodon. If Threads implementing ActivityPub means someone on Threads can follow what I post on my own personal Mastodon instance that's a good thing. It will increase my potential audience w/o me having to sign up to Threads myself.

Likewise if someone I'm interested in posts on Threads I can easily subscribe to them.

It's just RSS. I don't care what RSS reader you use.