There is nothing wrong with YEC as long as it is self-contained. The problem comes when it confronts or is confronted by science. The age of the cosmos has no bearing on sin and salvation or practical Christianity. But, if you make it important, say as a litmus to the accuracy of the Bible, it becomes critical.
The problem is not with the specific view but how it impinges on everything else. And, if you are going to attack that view you have to be able to replace the parts it underpins or the whole faith collapses like a deck of cards. IOW the reason so many young YECs lose faith is because they lose faith in the Bible. They have been shown that it does not say what they thought in terms of creation so they no longer believe any of it. All or nothing!
Before you can teach against YEC you must first establish that the Bible is not an authority on science. And that is easier said than done.
As a preacher I must allow for people to still believe in both a seven-day creation and an historic Garden of Eden with all the apparent incongruities with science or reality still intact.
Otherwise I am destroying not building faith. I can offer the alternative viewpoint, but must emphasise that it is not essential for faith in God or salvation.
Richard