Also, the non-inflated rubber wheels I had on the thing were very uncomfortable (and loud) on cobblestone-type roads. Almost infeasible to use, really. But otherwise it was a pretty convenient thing to have.
There are push scooters with inflatable tires (I've seen them here in the US) but not sure how much that affects the weight and portability.
https://www.kwigglebike.com/en_US/faltrad-300km
Ok, 300km/d is not for all people, but it show the possibilies.
Wheels are small, board is short, and center of gravity is high. It just takes a small pothole covered with leaves (so, not even winter), and the driver will fly forward (I did).
E-scooters are heavier, with larger wheels and longer boards, so they may be less accident-prone, but I've never tried one.
Another thing at this form factor you can get electric skateboard, muscles are concern.
If I haven't rode both bicycle and skateboards for distance for a while, I'd have exact same thought as you. From my experience of casual rider, it's opposite - I can easily go for hours on skateboard, while on bicycle I get tired and uncomfortable. I've ridden same bike paths 10-20 miles on bicycle and skateboard, and for me skateboard is easier. My guess because on skateboard I can change body position and muscles involved, can switch legs, push or pump. While on bicycle I'm locked in the same position, using same muscles all the time.
Jogging is way way harder than skate, I can barely jog 5 miles, but on skateboard 5 miles i won't even sweat.
There're various types of longboards and indeed some are quite high and you end up doing one legged squat. There many low to the ground models designed for distance specifically, that are much more comfortable to push, no squatting. Check https://www.instagram.com/p/Cbah6FrJuUv/ or https://www.gbomblongboards.com/longboard-skateboard-gallery
Cycling is more efficient that skateboarding, but surprisingly by a small factor. 24h record for cycling is ~900km, while on skateboard ~500km.
However, the limit on how good a bike is are defined by materials and geometry together. With strong, tight and shock-absorbing enough materials, you could theoretically create a race-quality bike in a form-factor like this.
When or if such things will appear is hard to say. The Bike Friday seems to be at the quality of a decent road bike but foldable into a suitcase (they've had the same basic design for twenty years so twenty years of materials improvement might create a pretty extraordinary thing).
See: https://bikefriday.com/ (relative to other comments, slightly larger than Brompton but I think more of a bike that really can be your only bike).
I can't see myself on one of these things but I never had a commute where it would be necessary, in a pinch maybe it's the lesser evil.
Longboards I think still have serious usability/safety issues compared to scooters and especially bikes.