I wasn't initially taken by that phone, as the industry has evolved towards less moving parts, and this mechanical concept doesn't seem to be the way forward.
The technology behind the screen itself will be used everywhere at some point (eg. curved adveristing boards etc) but the mechanics of the phone just don't seem to tick boxes.
However, when I saw how it works so well with videos, I became more interested. The way it could resize to specific ratios (eg. 4:3, 16:9) is great.
I'm not sure if I'd really require a phone-to-phablet device. Is this something that would take off? I'm not sure.
Myself, the option of a mini-tablet that transformed to a full-size tablet would be a much more appealing. I like the idea of having an ipad mini sized device (which is a great size for most uses) that could expand to the size of a full ipad for videos, and at a specific ratio.
The clincher is the screen quality, it would have to be full HDR10+, AMOLED etc, or I'd still prefer a standalone larger tablet.