First of all, a D160 from a Deere dealer is built EXACTLY the same as a D160 that is sold through Lowe's or Home Depot.
The D Series are entry-level tractors that are designed to sell for a specific price-point. So yes.... the components chosen for such tractors are not as strong as components used on the X-500 or X700 Series.
If the tractor has 12 hours on it, then it is still under warranty UNLESS it has been damaged by some sort of abuse, such as ramming into something that did not move out of the way. The dealer will investigate and find which part or parts are causing the problem. That part/s will be examined to see if it failed due to a manufacturing defect or from collision. If a manufacturing defect is found, then warranty will apply. If not, then the customer must pay the repair bill.
There's nothing new in any of that.
So, let's not blame John Deere until the evidence is in. If you want warranty, then the dealer must do the repair. That is how it works whether it's a car, truck or lawn tractor.
Comparing an entry level lawn tractor to a Gravely garden tractor is like comparing a F-150 Ford pick up truck to a Mack dump truck. They are not in the same league nor in the same ball park price-wise. Whomever bought the D-160 got exactly what they paid for. If it isn't robust enough for the job, then trade it in on a $10,000.00 X-700 Series.
It is up to the customer to choose the correct tool for the task at hand.