I don't see any reason why Sin 2 would destroy SA or any other Sin related mod.
First of all, Sin Arena is supposed to resemble Sin
, not Sin 2 or any other game. Note that it is a modification and not a full-blown project like Sin 2 would be. Also, you won't need to spend any cash for Sin Arena; that means nada, none, nothing. All you need is a copy of Quake III. And lastly, it's not like Sin Arena and Sin 2 will be out at the same time anyway.