On the one hand, the line is, "no, the purpose of ethics is to define 'ought,' to set out the ultimate picture of perfect human behavior, of what a human life could be at its best." To lower the bar would be to undermine the very purpose of ethics, we want to know how to live a good human life not just a good enough human life.In this case, both alternatives implicitly accept a priori that there is some ethical ideal, "the ultimate picture of perfect human behavior"; the question is, can we realistically aspire to this ideal.
On the other hand, the argument is that ought needs to entail can. If we are talking about how people should live, then it must be possible for people to actually live that way.
But "the ultimate picture of perfect human behavior" is a notion for totalitarians, however well-meaning. There is no ultimate picture, no best human life. There is only the life each of us wants to have, according to his or her own subjective desires. "One man's meat is another man's poison;" what's good for me might be terrible for you. But why should we choose?
If nothing I one person did could possibly effect another's happiness, there would be no need for ethics at all; the best we could do would be to give each other optional advice, and advice isn't normative. Sadly, our actions do affect each other, quite a lot of the time, generating conflicts. And thus the study of ethics, then, becomes not the search for perfection, but a search for ways to resolve those conflicts.
What's the best price for a car? The answer is different for the buyer and the seller. The notion of the "objectively" best price for a car is incoherent. How much does the buyer want the car? How hot is seller to get rid of it? There's no sense in which any price can be considered objectively optimal, but whatever price the two agree on is, by definition, good enough.
In the same sense, ethics are always about conflicts, where one person's better is another's worse. In this sense, there can never be perfection, optimality or any coherent notion of "best". There can be only good enough.