Its not just defog mate. Its ability removes multiple strats from working at all. Just because a pokemon doesnt instant kill everything doesnt mean that it isnt severely limiting.
since you seem so insistent that it removes multiple "strats" from working, how about we go over common stratagies and see which ones he
actually invalidates:
screens: doesn't invalidate
trick room: doesn't invalidate
weather: doesn't invalidate
hazzard stack: hm, let me think about this one
paraspam: makes harder but doesn't invalidate, you can still para gholden with hatterene nuzzle and the rest of gholds team is still vulnerable to para
perish trap: you could argue that gholdengo invalidates this but like, so does the entire ghost typing so its not like this is a trait unique to ghold
stall: ghold is just straight-up a stall staple, he's just that splashable
webs: doesn't invalidate
terrain: not unless you count it walling hawlucha to be invalidating (which I don't because you have 2+ other pokemon to abuse terrain with)
futureport: being a psychic resist doesn't count, doesn't invalidate
tailwind: does not invalidate
status spam + hex: again, makes a bit harder, but doesn't invalidate, especially given gholden is weak to hex
I genuenly cannot think of a single strategy that gholdengo straight up invalidates, and most of the ones that he poses a threat to wouldn't be good even with ghold gone, so please, do indulge me, what "strategies" are you refering to, and don't just use vague examples.
edit:
when 1/3 of all moves get completely invalidated by gholdengo simply switching into it then the problem starts to extend past just blocking defog. it's not just corviknight who gets bodied by this thing when all forms of defensive counterplay is completely invalidated whenever it simply enters the field
calling it 1/3 of all moves is a missleading statement as well as a plain wrong one, good as gold only affects status moves that target it directly, such as thunder wave or leech seed, it does not however, effect moves like swords dance or screens, this combined with the fact that the vast majority of status moves just plain suck means that in practice good as gold only blocks like, 7 moves that actually see use