I find it hard to trust a company like Microsoft to give away an upgrade (that supposedly improves a thing or two) for free without some catch. Do they guarantee full service and support? Will there never be a subscription fee for any features? Will windows 10 never pester me with any advertisements or force software on me that I don't want? Will all the features remain active indefinitely in the future? Will the new rolling release upgrade schedule never send my PC into some infinite upgrade loop or blue screen of death?
If I had good faith that the answer is "yes" to all of these questions, then I'd upgrade. But I don't have this faith, so I'll rather pass this upgrade until I buy a new machine or until there is some compelling reason to upgrade.