When Comcast launched its Xfinity Flex streaming box for broadband-only subscribers last year, it seemed a bit silly to pay $5 per month when you could just use a Chromecast-type device instead.

In April 2016, Comcast said Xfinity cable customers would be able to forgo an expensive cable box before the end of the year and use a Roku box instead. The company missed that target, but the good ...