See for me, a 17inch laptop would be too large, even a 15inch one is pushing it. Basically because I want to be able to easily use it on a plane, bus or train. Ever seen someone trying to use a 17inch laptop in economy?

And the larger the laptop the less additional stuff I'm able to put in my carry-on.
I think you're missing the point of the
desktop replacement computer. These aren't made to fit into economy class; they're meant for
1st Class!
Actually they're not meant for mass transit. Their primary purpose is to offer desktop processing power in a
AIO form factor. Some don't even have batteries. They're not supposed to compete with "clutch purse" style
notebook computers. They truly are the inheritor of the
laptop moniker.
I just got a
17" desktop replacement machine to take (on the road) with me for when I visit my mom. It fits nicely in a briefcase-sized
transit case, and gives me most of the comforts of home for my 1-2 week stays, where it's my desktop on the road, where there is no 21" monitor waiting for me.
If I had to take an airline flight (something that's increasingly unacceptable here in the US), I'd probably leave my DTR at home, pack my thin laptop and carry on my tablet-with-keyboard if I wanted to use a machine at all while in coach seating. Usually I savor the time I get to spend
without a telescreen staring back at me.
Speaking of transit cases, I actually have one of these:
A 4RU computer chassis on shock mounts. Although a keyboard and mouse could be stored in the back, adding a decent-sized monitor means another case. Adding a UPS/power conditioner means another case.

For some reason I haven't gotten around to taking this form factor on the road. I'll take something like this across town to do live streaming from. I know that I can turn my back on it and it won't walk away like a laptop might do.