Part of the issue with these sorts of console add ons is that they just split the addressable market further. ISV's either have to address only console owners with the add on, or they can address everybody with the console if they can manage without the extra hardware. Commercially, this can be a hard sell, which makes it difficult to sell the add on, making it even more difficult to write software requiring the add on.
You're being too positive. Only two add-ons in the history of console games have been successful: the Famicom Disk System and the PC Engine CD. All other add-ons have failed to sell enough to warrant vibrant development. Note that both add-ons were japanese successes.
No add-on has been able to survive more than one year outside Japan. Not even PS VR, whose sales are too low.
Despite its success, it still had a lower attach rate than the Famicom Disk System. In terms of software, after the first year no one could make any good money selling Kinect titles.
> Kinect for Xbox 360 continued its momentum in 2011 reaching 10 million Kinect sensors sold worldwide to date. Not only were sensor sales an overwhelming success, but Kinect drove significant game sales with more than 10 million standalone Kinect games sold worldwide to date
From 2011. At any rate, it obviously satisfies the grandparent’s comment on success. (Also, you may be thinking of the Xbox One, which they received flak for bundling with a Kinect)
Microsoft knew add-ons simply do not work long-term. That's why they put a Kinect in every Xbox One's box. Didn't solve the problem of everybody was over with the concept though.
https://en.wikipedia.org/wiki/Super_NES_CD-ROM
Part of the issue with these sorts of console add ons is that they just split the addressable market further. ISV's either have to address only console owners with the add on, or they can address everybody with the console if they can manage without the extra hardware. Commercially, this can be a hard sell, which makes it difficult to sell the add on, making it even more difficult to write software requiring the add on.