I don't know about you, but I can buy bows and arrows at hundreds of sporting goods stores in my local area alone, and I even know of 2 local blacksmith shops that sell swords.
Castles still exist as well, you just aren't invited to them (which was true for us peasants back in the day, too). Trump is still trying to get one built under the ruins of the East Wing, in fact.
The point is not that these cease to exist. The point is that their significance decreases greatly.