A store running a Sullust event of any size and not handing out prizes is problematic.
A store cancelling its Sullust event (even if they also run casual Armada games) is something that might happen.
Knowing how much the kits cost, an event that has 2 or 3 players isn't going to approach a break-even point for the store unless they charge enough that they're basically selling the ships. So I think there's a difference between just not handing out prizes, and cancelling the event due to lack of attendance.
That being said, if they get 4-8, they absolutely could run an event. They might have to increase their entry fee from what it would have been for 10+, though.