can't a malicious bluesky admin steal/MITM users' private keys by messing with whatever frontend javascript users interact with?

In theory, kinda, but you can bring-your-own client, and "the" web client is decoupled from the back-end instance.

"bsky.app" works as a web client for the official "bsky.social" instance, but it also works with the instance I self-host (or any other spec-compliant instance). Likewise, 3rd party clients work with the official instance, and also with 3rd party instances.

However, no key-stealing could possibly happen right now in any case because... the PDS ("instance") holds your signing key - the client never even sees it. Having the server hold your signing keys is very user-friendly, but of course not ideal for security and identity self-sovereignty. In general, the security model involves trusting your PDS (just as you trust your mastodon instance admin, or twitter dot com - the improvements are centered around making it easier to jump ship if you change your mind).

Client-signed posting is something that's not even possible right now, but I believe it's somewhere on the roadmap. If it doesn't happen some time soon I'll be implementing it myself. (I'm writing my own PDS software)

Yes, at the end of the day a malicious client is always a risk with this sort of thing. But the AT Proto does have some mitigation in place—users have a signing key which their PDS needs to act on their behalf (sign posts, etc) and a separate recovery key which users can hold fully self-sovereign and use to transfer their identity in case they detect malicious behavior. It's not foolproof of course, nothing is, but it is thoughtfully designed.

But yes, the protocol does have a fair bit of trust of your PDS built in. But that's inevitable for decent UX—imo the crypto craze proved that basically no one wants to (or can) hold their own keys day-to-day. If you want to have a cryptographic protocol that the average person can use, some amount of trust is necessary. The AT Protocol artfully threads the needle and finds a good compromise that is a (large) improvement over the status quo, in my opinion.

How is this better than everyone having their own Wordpress or Drupal install?

That's never going to work for the average person, sadly. And it misses a lot of social features that a lot of people (myself included) want from social media. Simply put, the UX is way too far off what people want and need.

It will, ISPs just need to start providing the basic hosting infrastructure on their routers again, like they used to. Thankfully we're also at a time where IPv6 is mature enough so that this is greatly simplified !

WordPress is not exactly known for its security.

Sure it is? WordPress updates itself and all plugins automatically. I've had Wordpress sites running for over a decade with zero security concerns ever popping up.

Maybe your point of view is outdated?

Wordpress doesn't have ActivityPub built in, it's a plugin in beta currently. Without AP, there is no client that can pull in website feeds and provide discoverability between WordPress sites, Mastodon posts, etc.

Back in the old days, activitypub was my Rss feed reader. Discoverability was driven by good old fashioned cross linking, comment discussions, and skimmable feeds from aggregators like the one we're on.

People love to reinvent the wheel and claim it's a whole new thing. No ideas on the web have really been innovative since the bubble popped. The innovation has all been on delivery and execution (not wanting to discount any of that).