I agree with most of what you said in general, but I'm talking exclusively about the Xbox One and what it provides above and beyond simply plugging a USB into a Smart TV.
The Xbox One outputs exclusively using HDMI and most (if not all TVs) with HDMI will have basic USB functionality. Again I'm not talking about anyone that owns a TV, I'm talking specifically about what Xbox One owners.
I'll give you the last point about the Smart TV features but will disagree with everything else
I'd even call fault there, most if not all HDTV's have had HDMI for half a decade, or more..
I mean I got a TV a few months ago, it's my bedroom TV, good brand, 3x HDMI.. no Media function or anything.. USB is just for updating firmware.
Most new High End TV's do have their own mediaplayer and such, yes.. But those are still a very small fraction of TV's sold.. most are no-name TV's from walmart and such, followed by the lower end models of the brand names, which also lack those functions.
In a Decade or so, you'll be right.. But most current TV's sold have no smart TV features, and the vast majority of TV's sold before lack those features.. Until most all TV's sold have the features built in, and until people need to replace their old ones, I'll still put Smart TV's as a minority.
Also as for why XBox One over just using your Smart TV (assuming it's player isn't ######).. well, XBox One also will play your DVD's and other stand alone media, it'll let you play games, etc.. all without signing out, changing devices, changing remotes.. It's convenience.. Not to mention the XBox One will get updates, which means in theory it's support of formats should carry on for years, vs a Smart TV that gets only the most basic update, rarely any new features or new support (no, they want you to buy a new TV for that stuff..)