Rim brakes need frame reinforcement at their mount points too, albeit not adapters, so that’s 20g. Hubs are 20g heavier each… I wouldn’t really go for a 20 spoke wheel myself but if you did I suppose that would save 20g. 50g for thru axles. 120g on the brifters.

So to add it all up you’re looking at 770g in components/additions, offset by 75g skewers, 310g calipers, and an added 25g or so per rim for the brake track. So discs are naively something like +335g. I obviously agree it’s a heavier system, but I don’t think ballparking the penalty as triple what it actually is is reasonable.