The problem is that Windows defect breaks laptops not under warranty by Microsoft. Getting refund would be tricky, and voiding Dell warranty in such case seems justified.

You could argue this for pretty much anything? Your mac screen failure is LG's fault, not Apple's. Your Asus GPU's failure is Nvidia's fault, not Asus's. Your internet outage is Cisco's fault, not your ISP's.

Ultimately the company selling to the end user has to stand over the package they're selling.

The problem is that windows is bundled, so they are definitely responsible for putting defective software on their defective laptop. Putting suspended laptop in a backpack cannot void warranty. This is like from another universe or what.