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I'm surprised I don't see other similar requests, but it would be a big help if the Roku remote was able to switch inputs on my TV. When I'm done watching streaming channels and want to switch back to cable, I have to either use my tv remote or just use a third party replacement remote.
There's no "but". Adding a learning feature for two buttons is easy to do and easy to use. It's certainly no more difficult than the current method of "programming" those two buttons. There's no reason the feature can't be added if it can be done on a $20 replacement.
The Xfinity Flex remote has a button that controls the TV input. It worked just fine with an old Panasonic plasma TV with a very simple on-screen setup. And the remote is no bigger than the Roku remote.
In that case, put in a new suggestion to request that future roku remotes have more learning features, as a clearer request.
This would probably require a partnership with someone like Oneforall (wouldn't be a bad idea for Roku to outsource remote production to them anyway).
I stand by my view that learning remote solution isn't a universal or 100% reliable solution in most cases. It may work for many, many others will find it "too hard", and a for a small percentage it won't work acceptably. A quality roku branded HDMI port switcher integrated to the streamer would be plug and play bulletproof reliable and easier to set up, and would put roku more front and centre in the home AV experience, especially versions which offer Bluetooth audio device connection (and maybe assistant apps in the future from Amazon, google or even Roku).
NB you might also want to read up on recent history of Logitech harmony, especially the blog post written by Matt Stoller.