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What’s more, this could be also regarded as a way to encourage users to adopt the newest operating systems
Considering many (most?) users buy pre-built computers, that was already true. When a new OS gets released, the previous one vanishes from this kind of market rather quickly and unless you are building your own rig, you'll have a hard time finding one that still comes with the older OS.
You can still connect to and download from Windows Update on really old operating systems.