There was a post on the UKNHCP forum a few days ago about a horse which had been barefoot for some years but had weak frogs.
The owner had tried lowering the heels by trimming, but actually frog strength is more about correct movement and stimulus.
It reminded me that we have often used these 2 photos in talks to farriers and students; they provide a perfect illustration of what makes frogs improve.
The right hand photo shows the hoof of a barefoot youngster; his hooves are fine, but under-stimulated because at that stage he was not in work and was just turned out in a field.
In the second photo its a different story - he had been in work for 6 months and the frog development is dramatic. He is now a trainee endurance horse, and is clearly landing correctly, heel first, and is moving over all sorts of terrain competently.
Its not the heels that have lowered, its the frog that has developed :-)