If I have both Thantis, the Warweaver and Koskun Falls on the battlefield, and it's an opponent's declare attackers phase in a multiplayer game, can that opponent tell me, "Oh, I want all my creatures to attack you, but I don't feel like paying the mana for it, so they don't have to attack at all"? If so, do I get to say, "You don't have to pay mana to attack anyone else, so your creatures must attack one of your other opponents besides me"?
In general, if you control Thantis and Koskun Falls, so that, among other things, each creature is required to "attack each combat if able", the creature must attack another player or a planeswalker if able if the creature doesn't attack you because the opponent declines to pay the Koskun Falls cost (C.R. 508.1d). See also this thread.
The players will have to obey as many requirments as possible. They don't have to pay to attack you but they could still attack another player and by doing so obey one more requirment than if they don't attak. Therefore they would have to attack someone else (or pay to attack you)
508.1d The active player checks each creature they control to see whether it's affected by any requirements (effects that say a creature attacks if able, or that it attacks if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of attackers is illegal. If a creature can't attack unless a player pays a cost, that player is not required to pay that cost, even if attacking with that creature would increase the number of requirements being obeyed.