Echoing all the comments above:
I've never purchased HD content deliberately in my life (sometimes you don't get a choice now).
I've never cared about anything but a reasonable screen size given the purpose. This means, yes, a smartphone is MORE THAN GOOD ENOUGH to watch HD movies on. Which is it? Is SD too blocky that I would notice on a phone, or is the phone good enough to watch HD on from the short distance you would. Also, my 32" TV is perfect. Doesn't consume a wall, while being great for gaming, movies and TV.
Strangely, the money I've saved on not having stupendous display devices is spent on ACTUAL CONTENT to watch instead. Weird that. It's almost like I prefer actually watching things to measuring contests.
Additionally, hello, this is the 21st Century... Chromecast / Firestick? You just flick the app and send it to the TV. Done. Like hell am I booting up a full machine just to do that when the phone is next to me.
And then, when you really get to it, all the modern HD-only junk is really not my thing anyway. I much prefer to put on an old sitcom, anyway. Who cares whether it's SD, HD, or not in that instance? You literally NEVER NOTICED until someone brought out DVD's, then you NEVER NOTICED until someone brought out flat-screens, then you NEVER NOTICED until someone brought out HD, then you NEVER NOTICED until someone brought out 4K or whatever. Yes, VHS sucked and was blurry and we all knew it, even back then. But ever since, I can't remember complaining once.
Revisionist history of resolution/quality is what you suffer from. When you got your HD TV were you just thinking "God, if only this was 4 times more high res, I can't stand watching it?" No. And the reason is because you couldn't see it then and can't now. Same for SD/HD.
The best TV picture I've ever seen in my life, that literally made me go "wow" and use it for everything, was with a Hauppauge WinTV card on a SVGA monitor. It was so fabulous a leap in quality from an old interlaced TV that it was amazing, despite still only showing an SD signal (but it was pixel-perfect because of a huge loft aerial and a monitor that could show the full TV res in a tiny little window while you worked). Since then, it all looks the same at any reasonable distance (which could be inches for a smartphone resting on your chest lying in bed, for instance).
To me, I believe in the XKCD cartoon - HD is only slightly better than the monitor I had in the 90's, which I used every day a lot more than I ever did the TV. Only in recent years have TVs and monitors been the same tech in different shaped boxes. But I'm used to that kind of resolution. It's essential when doing pixel-perfect placement. But watching a movie? I really couldn't care.
Yes, a smartphone on my chest shows me a movie in all the quality I need. And I saved £2 by not buying the HD version. And I enjoyed the core point of the whole exercise - the movie.