I just got a foldable. The increase in functionality of reading on my phone is substantial, and that's such a big fraction of my phone use that I consider it worthwhile. I wouldn't be as productive if I had to carry a bulky e-reader with me all the time, it's incredibly convenient to be able to fold it up and put it in my pocket.
It's a bit heavier, but I got used to it quickly. My old phone feels suspiciously light now, like a toy. The expense is certainly a factor, but for me the utility is worth it in the long run. It's not for everyone, but there are people it makes sense for.
I dabble in comics and manga too, so I prefer the full screen to an e-ink display. It's also nice for videos.