Nothing has to be built stronger for discs, unless the braking force that they can provide is increased….assuming you have the grip available from the tyres to use that extra braking capability. The bottom of the forks may have to be stiffer, but the crown of the fork doesn’t have to be as strong as no caliper is being bolted there, so you gain a bit of weight in one part of the fork, and loose the same in another.
When you are braking you’re trying to twist the rim around the hub. This is the same with disc brakes as for rim brakes, the spokes will not see a difference.
The thing I struggle with regarding disc brakes on road race bikes, is that you’re limited to the grip available from the tyres. In can easily lock up with rim brakes, so the extra braking force of disk brakes is useless. Also from an aerodynamic point of view I can’t see how you can make a disk setup as aerodynamic as calipers which can be faired in on sculpted carbon bikes, especially TT bikes.
But for normal recreational bikes disc brakes make sense. They’re better in wet conditions, easier maintenance and with bigger tyres with more grip, offer better braking performance. Horses for courses, there is no one size fits all solution.