This submission is about BlackSky doing the whole stack themselves. There are other single node app-views. So, thanks for asking, with apologies, let me correct you: you are wrong.
There are lots and lots of full network relays. A couple that run a full network relay on a < $5/mo VPS . See my other comments in this thread, https://news.ycombinator.com/item?id=47302514
If all you want is like and replies (which is the biggest huge part of the app view responsibility), Constellation has done that, is open source, and can do a full network link indexing on a raspberry pi. And which runs public endpoints you can just use, that many apps rely on.
Atproto unlike Mastodon also faces these challenges that, if you tried to do them on Mastodon, would get you screamed at and banned from instances. Fediverse broadly doesn't want you to use "their" data. You can't write search tools. As a result, Mastodon doesn't need anywhere near the complexity, because these unofficial but vociferously enforced terms-of-service disallow thorough interconnection to begin with. Makes it technically much simpler to pull off! No one is allowed to get a full fire hose. No broad app views are allowed. Dunno if this is still true, but for the longest time you wouldn't get likes or comments from someone unless they were already followed by someone on your fedi, and it's a direct result of this deliberate explicit lack of interconnection.
Those constraints greatly reduce the difficulty of scaling Mastodon: technical scaling is not a problem if you socially don't allow scale. So distributed you can't even read it.
Doubt is allowed. But from my view, Mastodon world deserves the doubt. It has stood still, barely budged. I'd love to be wrong. But it seems dominated by the one software that is Mastodon, and it doesn't seem to have a universe of interesting connected neat social softwares sprouting up left and right. The software centralization is near to total, the protocol centralization even worse. As a result, there is little distinguishing interesting novel Mastodon technology happening. The one API is deeply rooted. ActivityPub is trying to find some way to get started breaking this mono-culture & enable innovation but that's just started.
Meanwhile a casual glance at Atproto shows hundreds of amazing apps and systems and clients springing up, from amazing empassioned developers. 2025 saw a massive amount of technical decentralziation. Npmx and Eurosky setting up sizable idependent public/public-ish PDS instances, and has shown people indeed moving their core identity off BlueSky servers at some kind of scale. I forgive you for not knowing how far things have come, and I forget myself how incredibly quickly it's come together. It's still 99.99% Bluesky concentrated hosting, but it is distributed, has independent services for the full network, there is credible exit (1000 moved to npmx hosting in the past ~3 weeks), and to me the most important thing: there is technical diversity & independent exploration. There are so many devs building amazing things. That feels so so so absent on Mastodon.
That's the checkbox at the end of the Fermi great filter of interest for online social systems for me: can we permissionlessly build interesting social systems & experiences with these social protocols, or will each system have to look like a cookie cutter copy of a single instance?