What is the concern? Nobody can steal it from you and get away as there's only a few spots to hide at 37K feet. I'm a frequent flyer. Sleep masks are COMMON. Nothing happens to the people not just hiding their eyes from their surroundings but opting to be unconscious while flying too. Sometimes sleep mask people are in headphones or buds too. No consequences. Nothing happens to them. Sometimes sleep mask people in buds or headphones have a huge inflatable donut pillow wrapped around their neck to keep their head upright. Again, no consequences.
People carry sizable carry-ons on every plane, often with another bag loaded full to stuff under the seat in front of them. That bag is usually full of various kinds of tech. Vpro could just be another bit of tech in a bag like that.
As to 2.5 hours of battery, buy & bring an extra battery or two. I lived on a couple of batteries for flights for years with the old Powerbook G4. No problems. No consequences. Just switch to a new battery when he existing one is running down. Or plug into power when it is available at the seat.
For me, I'll be bringing BOTH MB and Vpro if I buy Vpro. Vpro will be the super-sized monitor while I use the MB's Mac horses for work during flights. For others who may not need full Mac apps and/or get by just fine with iPad-level apps, maybe they carry only Vpro with a spare battery(s) and maybe a
roll-up/
un-folding or
unfolding with trackpad bluetooth keyboard if they want to type a lot of stuff on a real keyboard.
In the longer run, I'm hoping I can buy a lid-damaged MB, remove the lid, leaving only the bottom (Mac) part (keyboard and trackpad) with Vpro being its monitor. This already works just fine by attaching the bottom of a lidless MB to a regular monitor and I'd love it if it could work the same way with Vpro.
Dongles & cables are carried because someone MAY need them but can't know they will need all of them on any given trip. Frustration is at carrying a bunch of "maybe need" stuff. Those opting to use some combination of MB and Vpro know they will be using both together. It's a very different proposition.
However, those concerned with minimal weight/load will probably NOT buy Vpro to use with a MB and instead carry only the MB itself and use its 14" or 16" screen.