I'd rather that ATProto was just compatible with activitypub to begin with, or allowed for meaningful federation.

Bluesky structure is too different from Mastodon instance to have a direct link. Brid.gy is the link between the 2 networks. Isn't is enough already for federation ? From the AP side, it's just a mastodon instance. However by default it's not open to everyone, Bluesky users and Mastodon user first need to opt-in to be able to interact.

Boobam, a japanese blogger did a report / comparison of Bluesky with Mastodon. One of his conclusion on Mastodon, is that it can't grow too much, by design. The more Mastodon traffic grows, the slower is gets and smaller instances will be overwhelmed. Now, there is no defined limit from when Mastodon will slow down so much it will be unnusuable. You can use your favorite translator to read the page: https://whtwnd.com/boobam.bsky.social/3l5uqcwjawe2e

There is less of defederation in the email network, because mail has been build to reach its destination no matter the path or the time, message must arrives even partially. (this was a premise of the US military, at ARPAnet). Even if a mailadmin blocks one server, mail could go another route. This is also the base of the internet, path is not the most straight forward direct link between source and destination (companies are usually against this structure)

AT or Mastodon don't have this freedom or constraint (depend of one point of view).