If you know you have done something wrong, can you choose to live better from now on, or not?
That is what Erasmus also says, yes. He says again and again that humans can do what he wants to or otherwise that he knows that he is not able to do what he whishes. But such people do not exist.
The Bible shows us people who are not only bound, miserable, captured, ill and dead, but who are also (by Satan, their ruler) so blind that they see themselves as unimpeded, fortunate, free, mighty, healthy and alive. Because the devil knows that his empire would become empty should people really realise how things are. Because when they see how bad they are, they cry and then God can do nothing else than take pity on them and come to the rescue. After all, He always does that with the broken-hearted.
So the devil makes it so that people do not realise their misery. Therefore they keep on believing that they can really do everything that they have to do.
But Moses is sent to uncover and to reveal their misery through the law. He had to prepare men (broken and confused through their new insight in their misery) for the grace of Christ. That is therefore really serious and necessary.