Despite understandable misgivings with ATProto due to its corporate origins and its architecture lending itself to centralization, it's still open source. Moreover, it serves a different purpose compared to ActivityPub, in that it specifically aims to enable and support larger scale social networks.
In a way, ATProto could be complementary to ActivityPub, but for this to be the case, there needs to be more shared understanding between both communities. People working on both recognize the faults in existing social media, and aim to address them in different ways.
ATProto provides an opportunity to break down big social media enclosures with data portability and a similar vibe to big social media, but with more individual empowerment to adjust what they see. The latter point is a commonality with ActivityPub, but ActivityPub provides a different angle of breaking the big social media enclosures.
Where ATProto serves the interests of those into big social media vibes, ActivityPub serves the interests of those into small social media vibes. In other words, ATProto scales up, where ActivityPub scales down.
ActivityPub is arguably a better protocol for both individual and "small" group empowerment, as it can enable otherwise less active, small platforms to connect and ensure there's always some level of activity to encourage people to come back. Think of old forums that, on their own, gradually faded out as people stopped visiting and posting for more active online communities. ActivityPub can serve as a buffer against that, to some degree.
Together, both protocols could provide a better, open social web, and perhaps effectively topple big social media enclosures. After all, who wouldn't like to see the web without Meta/Facebook and Twitter/X?
TL;DR: ATProto/ActivityPub have a common foe in big social media enclosures like Meta/Facebook and Twitter/X and would be better served working together to erode their influence.
AKAIK, activitypub (on mastodon) only requests and receives content from individual users that have been followed by someone on the local instance, it wouldn't load all of bluesky at once, it would just need to have an up to date database of bluesky's users so they are easily searchable. With that model, even crappy PC's can interact with the mega servers.
With the OP's point of ATProto being open-source, I assume the thrust of the argument is that at done point there would be a community hosted instance, which were it compatible, I think most activitypub se rvers would gladly federate with.
Ideally there doesn't need to be any, as the conglomeration of all smaller instances should be able to act as a large server. Unfortunately as it currently stands, the UI of most fediverse software makes interacting with that wider pool more difficult than it needs to be, and thus punishes smaller Mastodon servers with more difficult discovery of interesting topics or people to follow. But I think that can be overcome simply with better UI design.
Again, I think that's a UI problem. I don't use mastodon myself because of it, as I find it difficult to find people that interest me. However, Lemmy's use of Topics, and more critically, the existence of Lemmyverse.net which searches across all instances, make finding interesting things possible regardless of the size of your home instance. It's criminal that that functionality is not a native feature in the standard lemmy Ui, and I'm not aware of anything similar for mastodon.
The vast majority of people landed on the largest severs, where there was a ton of chatter in both the Local and Global feeds. They couldn't find anyone because they were accustomed to using search to find connections, and Mastodon's search is so tightly locked down that it's borderline impossible to find who or what you're looking for. The idea of checking other feeds didn't even seem to occur to many people -- something that I, as an avid "what's that button do?" user, find totally bizarre, but seems to be rather standard for most people interacting with new things.
If the default view had been Local or something, the story could have been very different.