There are the odd moments when we have an abusive and hurtful Dom, or an absentee Dom that doesn't login anymore. Is the sub doomed?

No. Because in the Signature system, the sub is allowed to reset her owners (just say "/1.owner" and hit "reset owners").

Resetting owners resets all locks as well. Think of it as a guaranteed safeword (as in real life, a safeword is a must!).
This also prevents griefers from having any sort of permanent control over the sub, even if the sub gives "ownership" to a stranger.

BUT: For certain toys, there is a lock-down mode wherein the sub CANNOT reset her owners (and locks etc.) via the normal means, when and if she is locked down.  But this lock-down requires the sub to first attain the Slave Circle before this mode can be accessed - so subs do not have to reach this level if they don't want to. This choice is entirely up to the sub - not her Dom/Domme.*

BUT BUT: Even in the lock-down mode, the sub is not doomed. She can go to "Fingers", a shady bot in Lulu's Secret Warehouse and pay him to activate a one-minute window for the sub to reset owners (and automatically, all locks). Fingers' prices vary over random periods - it can be very cheap, or not so cheap. So the recovery option is there, but because there is a cost (L$ and time and risk of missing the one-minute window), the sub is quite unlikely to make resetting owners a regular habit.

*For those who don't want to give up control but would still like to progress to higher Circles, they can either (1) be entirely self-owned, and use the toy's Promiscuous Mode (e.g. Signature Cuffs) to let others play with her, or (2) assign an owner, but have an alt as another co-owner. This way, you are protected without having to look for Fingers, though the flavor is not quite the same.