4chan isn't /b/ alone. There are plenty of niche boards which are worth hanging out on if they're relevant to your interests, without being the toilet that is /b/.

The porn and violence started primarily as satire.

They didn't stay as satire.

This has led to the follow theorem of mine, which describes /b/ perfectly:

Any community that gets its laughs by pretending to be idiots will eventually be flooded by actual idiots who mistakenly believe that they're in good company.

I guess the community (i say this loosely) changed drastically right around the "down with Scientology" stage.

The influx of people interesting in fighting Scientology had little negative effect on the morals/behavior of the people hanging out on /b/. I think you're right that the community changed, but not for the worse in terms that non-/b/tards would recognize.

Actual /b/tards didn't like all the unsocialized newcomers arriving and parroting memes that they didn't understand ad nauseam. They also didn't like the moral crusading aspect of the newcomers. Neither of these behaviors is a negative change in the community from an outsider's perspective, but it was from an insider's perspective.

The Scientology campaign didn't change the (sometimes satirical) celebration of racism, porn, fascism, child porn, misogyny, and so on--all that pre-existed the influx of new people interested in fighting Scientology. If anything, the new people watered down the sociopathic culture of /b/.