Just tell them why you asked

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After 3-4 topcase replacements:
1. The computer has been disassembled and reassembled that many times. They scratch the case, they scratch the display (at least in my case), misalign the hinge so it creeks. As everybody most probably knows by now - Apple devices are not meant to be opened and fixed. A new device will remedy all of that.
2. Enough time will have elapsed that there will be a newer model out, most probably with a revised keyboard (which unfortunately is still garbage).