Problem:
No Amish person could ever claim that they have never thought any wicked, sinful or evil thought after Rumspringa.
The “battle of the mind” - the war of the Spirit versus the “flesh” - is a war that the Apostle Paul says we as Christians will fight till the day we go to heaven.
It is a result of being born with a sinful nature. As Calvin, Arminius, Luther, etc all said in agreement with Paul on this topic.
Hence, no need for Rumspringa.
From the Amish point of view, it’s providing an opportunity for young adults to decide, do they belong to the world or do they belong to the Amish? Those who return are far more likely to be comfortable, accepting and committed to their unique way of life, and far less likely to create strife, division and dissatisfaction. For this reason it is a practical step for them to take. We are sinners always, so are the Amish, it’s just a matter of degree, and a matter of forgiveness. That’s true of every human being save Jesus Christ himself.
However, it is clear from experience that some people are more open to temptation than others and some are more capable of wickedness than others.
Rumspringa seems to be a good way to get rid of the worst apples.
"True Christians" maybe should want to keep those bad apples in their midst and try to get them back on the one true path and protect them from the temptations of the outside world.
The Amish seem to be doing quite well. The 'English' not so much.