Martin O'Donnell sent me a link to this article on DailyFinance.com, Roku Will Always Offer the Ultimate Set-Top Box, in which Sam Mattera suggests that Roku will achieve and maintain this distinction because it has the least conflicts of interest of any of its current or expected competitors.
According to the article, a set-top box from Amazon.com will feature Amazon Prime Video, Netflix, and Hulu Plus services, but not YouTube because Google will not permit it. This is because an Amazon Set-Top Box would use of a forked-version of Android as the box's base OS, just like the Kindle Fire tablets do. (That OS is known as FireOS.)
If you buy into that, then Google Chromecast is also crippled in a similar manner, because Amazon.com will not create an Amazon Prime Video app for Android. The existance of Amazon Prime Video as a native service on Android would remove one of the key differentiators of Kindle Fire tablets.
Roku 3 has an Amazon Instant Video client today, in part, because Amazon.com is not yet on the market with a set-top box that competes with it. Roku also has Netflix, Hulu Plus, HBO GO, Major League Baseball, and YouTube. This is why people who don't have a major investment in iTunes video content love Roku so much.
But saying that any of these companies will "always" do anything is preposterous. Roku can only be counted on to produce a set-top box with a wide variety of streaming service clients for as long as it remains an independent company and the bigger companies mentioned in this article continue to see the world the way they do today.
Apple may always want to have maximum control over the presentation of its services, but AppleTV as we know it today may not exist forever as a standalone box.
I've recently wondered what would happen if Apple offered to allow an Amazon Prime Video app on AppleTV, but not Amazon Instant Video, which is a direct competitor to iTunes video in many respects? I don't think Apple will offer Amazon this opportunity, and I don't think Amazon.com will accept it if this distinction were made about its video services; Nevertheless, it would be an interesting offer for Apple to make.
I don't know what to say about Google or Amazon.com exactly. I use services from both companies and like them. But both companies may not always be willing to take the approach of giving away services at no direct cost to consumers, or providing these services with huge subsidies-- like they do today.
I don't think Roku goes the way of TiVo. But Roku is a darling of the technology press today more for what it isn't than for what it is.