That is not true. almost every single tavern owner (quest related and not) give a reward of at least one card. So that means that these unaccounted for quests (simply go beat them and win a card without a quest) will give you quests. In fact you can find all cards this way or you can find the majority by buying them from the vendors. Just like recipes for alchemy. You can either find them or you can buy them.
My point is hunting through the world and beating 50 vendors is not very interesting or challenging and they are not actual quests. Each of the areas has a players of gwent quest so you still get to enjoy the challenge of playing various people in the world just not dozens of them.
If they do let us keep our world cards then it would be good to upgrade opponents in NG+ by giving them 1 or 2 'rates'.