Relays are actually quite cheap now! They used to be a bit more expensive when they archived all the traffic, but in sync 1.1 that was dropped and they can be run on $20/mo VMs pretty trivially now
Someone has to archive the data, right? Perhaps the complexity has been pushed to the AppViews?
If I read a post from a month ago, how will my client know how many likes it has or what the replies were? Without reverse links, someone has to look up in a database of all AT actions for likes/replies pointing to that post.
Contrast with the inbox/outbox forward/reverse linking model of activity pub.
AppViews already had to archive whatever data from the network they cared about. The change just means that an AppView won't be able to infinitely backfill from any relay, you either need to find a relay that archives all the data you want and backfill from that relay, or you'll need to backfill by querying pds's directly. Or you could just not backfill at all, your AppView can do whatever it wants to really. Clients always talk to an AppView to get information about the network, the AppView manages the building of reverse links, the relay doesn't care about the hydrated state of the network, its just a stream of events which occur on the atproto network.
If I read a post from a month ago, how will my client know how many likes it has or what the replies were? Without reverse links, someone has to look up in a database of all AT actions for likes/replies pointing to that post.
Contrast with the inbox/outbox forward/reverse linking model of activity pub.