If you've already cut the roots & moved the
plant to a smaller pot, the
plant should do fine.
(And you don't really have an option at this point anyway, but to hope it's OK!) You may have some of the leaves die off, but that's to be expected... just keep grooming it until it settles down & starts using those of the roots that are still attached, & it should flourish again.
I think that what I would have done before cutting into the root ball is that I'd have dunked it into the kitchen sink & tried to separate the roots, in case there's more than one
plant there. If it was indeed one
plant, I wouldn't think that thinning the roots would really hurt it much. (I would be more concerned about roots that are no longer attached to the
plant after "hacking", that might decay inside the soil, thus making the
plant vulnerable to disease.)
I don't know for sure if Spiders can get so big that they simply come to the end of their natural life, or not.
As you probably know already, an "overly rootbound spiderplant", will make babies like crazy. If they DO have a limited life span after they get super huge, the best bet is to root some babies in water & them
plant them in soil & start over from scratch!
Please come back & let us know how your Spider fares!
(By the way, I love this: "saving one pet wont change the world, but surely the world will change for that one pet....")