I chuckle at the thought of going back to letterboxing with people arguing that “we need to see the top and bottom of the frame, like the director intended!” What’s old is new again. Are hemlines up or down this year? Skinny jeans or baggy?
My desktop is 21:9, my Macbook and Steamdeck are 16:10, my TV and Nintendo Switch are 16:9, and my iPhone is some weird aspect ratio. So it's not like I'm not used to varying formats. And I actually like it when movies take liberties with the screen format instead of forcing everything into 16:9 just because it's the common TV format.
We also have two things working in our favor for home video releases on 16:9 displays. First, 16:9 is geometrically well-positioned between 4:3 and 2.39:1, so displaying either of those pillar/letterboxed on a 16:9 screen only leaves 25% of the screen area unused. (We literally chose 16:9 because it's the geometric mean of 4:3 and 2.39:1.)
Secondly, a typical home TV now is simply much larger than a typical home TV 30+ years ago, so the reduced size of a pillar/letterboxed image is still just straight up bigger than most home TVs were prior to the flatscreen revolution.