There's evidence that Bret had recently suffered a concussion before wrestling Goldberg. He continued to wrestle. Then another one. Then another five in the next six weeks after that.
Bret himself wrote that it was multiple concussions going all the way through January that meant he had to retire. He wrote that in his retirement message.
Independent sources corroborate all this. From the F4W newsletter #280 (nov 6, 2000):
>Instead of taking time off to get evaluated, Bret stayed on the road, taking powerbombs from Sid and wrestling hardcore matches with Terry Funk. When the headaches became unbearable, he finally visited his doctor and was given the bad news — he’d suffered a half dozen concussions within a period of just weeks.
Yes, the kick from Goldberg was a problem. But most of the damage was done after that. From the Wrestling Observer Newsletter, nov 6 2000:
>There have been doctors that have surmised he actually had suffered a concussion in the ring before that match, which magnified the damaging effect of the kick on his brain. But in any event, his continuing to wrestle, and suffering more head trauma, particularly in one of his final career match, a hardcore match against Terry Funk, led to his suffering about ten percent brain damage, some of which may be permanent.