I will never use the same service to sign into multiple things. It's a single point of failure. If Apple ever decides to close your account you're S.O.L.
I made the mistake once, had an account closed, got royally screwed. It wasn't Apple but it doesn't matter. I learned my lesson. Don't tie things together.
I don't follow that for everything but for anything important I do as well as anything involving money.
Just to expound on what you're saying, won't there always be a single point of failure? For example, for the majority of people there are only a few options.
1. Use the same password for all logins because you don't know how to manage unique passwords for all your logins. Obviously this is about as unsecure as you can get.
2. Write your unique passwords down somewhere. This can be in a notebook, or a password manager (1password and the like). In this case, there is still a single point of failure (as you pointed out) if someone finds your book or compromises your password manager.
3. Use some sort of SSO service. Still a single point of failure (Apple, Google, Facebook).
I feel like using Apple SSO with 2-factor authentication is just as secure as any of these options.
Is there any "secure" system that doesn't have a single point of failure?
You. You are the single point of failure; if you are compromised, then all your accounts can be accessed by the compromisor.
If you're looking for a point outside yourself, then memorising all your passwords would be an option.
But beyond that, I don't think your criticism is warranted. There's always a single point of failure - sure - but we can still consider gradations of how centralised that point is, and how likely it is to fail.
With a hosted password manager, you're at the mercy of their server code; specifically, at least for 1password, I think they have a 'dead man's switch' which lets you get at the encrypted content without the master password. This is more likely to fail than a password manager which stores all its content locally and really encrypts it (e.g. keepass). In this case, human error outside of yourself can't compromise you. But technical error can, which is why there are more steps that can meaningfully increase your level of security. Like running your password manager on a separate, air-gapped computer; or sandboxing everything you run a la qubes.
Are any of these especially likely to compromise you, as a user? No, but reducing centralisation and dependency still improve your chances, and are definitely worth considering if you are e.g. running a drug smuggling ring.
Most people are already vulnerable to this because they use the same password everywhere. The fewer service providers there are holding a copy, the lower the risk of compromise.
This is not how it works. We don't hold any copies of users' passwords these days, there are hashes for that. Certainly, some old, or poor quality in-house software still do it, but then it won't offer you integration with whatever SSO service anyway. And single point of failure is very real, if you trying to operate world-wide: sign-in with X may suddenly become illegal, or inaccessible outside of the USA.
I made the mistake once, had an account closed, got royally screwed. It wasn't Apple but it doesn't matter. I learned my lesson. Don't tie things together.
I don't follow that for everything but for anything important I do as well as anything involving money.