I disagree from the perspective that a such a counter being possible means that the person is sweeping at the wrong time. Sweeps are not techniques that can just be done at any time. Sweeps don't initiate, they take advantage of an opponent's poor structure, standing leg, weak stance, or movement. It's not something that one should do in the same manner that jabs are thrown out to break through defenses. There is also a deceptiveness to sweeps that have to be used in order to make them effective.The best counter for "sweep" is "single leg". In MMA, almost everybody are good in "single leg". When you apply "single leg", you have to move in a get his leg. If your opponent just gives you that leg, it will save you 1/2 of your effort.
Here's are 3 examples of me executing a sweep during free sparring. Notice that I don't execute the sweep when my opponent has the ability to counter the me. I take advantage of his movement and use correct timing required for the sweeps (See Video here at the 1:15). The sweeps that you see in the video are my slow sweeps with no power because of safety reasons. If my opponent was having that much trouble with my slow sweeps there's no way he would be able to handle a well timed sweep at full speed and full power. Each time I did the sweep my opponent was at a bad position to initiate a single leg take down or any counter that involves grabbing my sweeping leg.
This is what I'm thinking. The MMA ring is "fall" friendly. A person can take a fall in an MMA ring that would normally be the end of the fight had the same fall happened on a hard surface like packed dirt, grass, or concrete. It may only be good enough to get a knock down and to wear the person down as he has to use some energy to get off the ground provided that they would actually try to get up.The question is does the sweep justify the risk? Maybe not.