That's a solid point! Much as I gripe about my iPad Pro not doing things I wish it did, Apple still sells a gazillion of them to people who think their vision is better than mine. They're a $3T company by knowing what their customers want.
I'd be tempted to give Roku the same benefit of the doubt except that the person here on HN who says they like motion smoothing is literally the only person I've ever heard of who did. My in-laws are far, far from tech-savvy videophiles. Last time we visited them I had to set their TV to the right aspect ratio. They'd been watching it that way for ages. And when a software update turned on motion smoothing, they called to ask me how to turn it off because they absolutely hated it. These are regular, non-techie people who just want to watch TV and have zero interest in tweaking or tuning a thing. They are not nit-picky. If it bothered them so much that they had me fix it, I've gotta think there are a lot of people who dislike it.
And because of all that, I think this decision is bonkers. It's a hugely polarizing setting. People who like it like it. People who don't tend to absolutely detest it. I can't imagine what advice Roku got that made it seem like a good idea.