I should mention that I've worked as an infrastructure engineer on both MMOs and GitHub-like enterprise-y services.

EVE is literally the only exception to "MMOs scale by horizontal shared-nothing sharding"; and that's why I mentioned the option EVE uses instead — namely, "vertical scaling of hardware" (i.e. having a really honking powerful single-master multi-read-replica DB cluster.)

In neither case is anything "clever" (i.e. inefficient for the sake of developer productivity / enterprise integration / etc.) happening. There's no CQRS message queues, no async batch writes, no third-party services halfway around the world being called into, no external regulatory systems doing per-action authorization, no separate "normalized data warehouse for OLAP, denormalized data for runtime" forking writes, no low-level cross-replicated integration between a central multitenant cloud system and individual enterprise-hosted tenant silos, etc etc.

> With so many different play styles and optional components (guilds, pvp, official forums, paid content, user made content, etc)

I think you misunderstood me when I said that there's a rut that users are guided into. The thing about MMOs is that the ideal user uses all the features (because the game incentivizes doing so, and because the more deeply and broadly users engage with the game's systems, the higher their retention / lower their churn will predictably be.) The ideal player is in both a party (or constantly switching parties) and a guild; has paid for all the DLC and regularly buys cash-shop items; plays every piece of PVE content you build; engages in PVP content and co-op UGC content all the time; etc.

Which is to say, for the ideal user, "the game" either works or it doesn't, because "the game" is the whole thing. Every feature needs to work, in order for the game to work. Because the ideal user engages with every feature. The SLO is, essentially, "can you do a completionist run through every bit of content we have." (If you're clever, and can make your server deterministic, you can create a completionist run as a backend-event demo file and run it in CI!)

And this is, in part, why MMOs are kept architecturally simple. Everything needs to work!

(And I don't just mean "simple" in terms of the backend not being a sprawling enterprise-y mess, but rather usually a single monolithic binary that can keep a lot of state in memory. I also mean "simple" in terms of as much of the game as possible being pushed to local, mostly-ephemeral-state client-side logic. MMOs are, often, a lot less "online" than one might think. It's very hard to "break" an MMO with a content update, because most content updates are to zonal scripts whose state doesn't persist past the lifetime of the in-memory load of that zone in a particular interacting device.)

With GitHub, their ideal users — of which there are many — can be individually satisfied by very small subsets of the system, such that they're still satisfying almost all their users even if one system is horribly breaking. That's what an SLO is "for", in the end: to tell you whether different subpopulations of users with different needs are happy or not. If you only have one "core" subpopulation, one ideal user, then you only need one SLO, to track that one ideal user's satisfaction. If you have more, you need more.