Some people can adapt to things fine, but don't necessarily ask them to put things together.
And yes, I make no argument that rented boxes are GOOD or very capable. They are limited, locked down, and suck, but working with them means pointing the remote and, if something happens, pulling the power cable and resetting. It does everything a casual user needs, and if it does not, they have Apple TV, Roku, FireTV, etc. Their TV might even have the media offerings they'd get with an add-on box.
There's a world of difference between a Tivo and an HTPC. A Tivo might as well be a cable box, it is, again, very streamlined, and basically does what modern rented DVR boxes do with a little more flair. Remember that Tivo built a business on DVR before cable companies even started offering their own DVR boxes. This has led to numerous return users who simply liked it and never looked back. Some got good deals on hardware and paid a one-time lifetime membership, which, depending on overall hardware expense, will still be cheaper than the equivalent service and hardware fees from the cable company after a few years, if not a year or two.
I help my parents out with computers. I will never set up an HTPC for them, never. There are too many little odd things that can just pop up that will require a phone call to me.
Setting up a Tivo is usually done by a cable company on-site visit, or those who are comfortable can just put the card in themselves. And if something happens, it's a phone call to Tivo or the cable company. With an HTPC, you are the support, unless you can tell it is a cable issue and then you call the cable company.
I'm not saying these "truths" are the law or anything, there are always exceptions, and I'm glad when I hear of people who are even computer or technology illiterate getting by HTPCs, but I do know many people who just view it as too much to juggle. And it's not even a technology literacy thing, many very technical people just don't want the hassle. I feel it is easy and I can make things just work, but even then, I know there are times things just stop working or something goes screwy. I don't mind.