A major justification for removing the previous content blocking API was that it could be used to do things like inject JS. So clearly the intention is to have content blocking extensions not do that at all. Although in this specific case, it might still be possible.
AdGuard and uBO for example use the content blocking API to inject blocking "scriptlets" on sites where this kind of thing is required. That kind of usage is made much more inconvenient with Manifest v3.
I don't think so. Injecting JS is a valid use-case and will work forever, probably a huge majority of extensions do that. An intention is to make content blocking extensions more performant.
It's very easy to inject JS. I don't know whether you're talking from your own experience, but I wrote my little extension to replace uBlock (with my own list of rules and blocks) and to inject JS or CSS you just have to add a line in manifest.json which have nothing to do with blocking API.
I know it is easy to inject JS and that you can do it with the manifest file. But without the old content blocking API you can't dynamically inject different snippets on different pages based on filter lists for example (unless you inject something on every page).
I wouldn't be surprised if in the future, content blocking extensions won't be allowed in the store if they use such broad permissions for example.
Well, to be fair, not really. Scriptlets will continue to work just okay.
To be completely honest, Manifest V3 technically is not THAT bad and it's capabilities at the current moment are really close to what major ad blockers can do.
There're still some things that bother me:
1. Debugging a content blocker is really inconvenient (not as bad as Safari though)
2. The future. What if its development stalls after it's released?
3. Google's goal (probably, for Manifest V4) is to make content blocking completely declarative, i.e. get rid of any host permissions and content scripts.
AdGuard and uBO for example use the content blocking API to inject blocking "scriptlets" on sites where this kind of thing is required. That kind of usage is made much more inconvenient with Manifest v3.