You're right about the coronary on the moped. I never had a gas-powered moped, but I had an electric scooter which was given to me for free. The pedals are utterly worthless. The gearing was too low, the cranks themselves were wiggly, the stroke was too short, and the riding stance was far too wide to generate enough sustained movement to balance. I think the farthest I got it to move on pedal power from a standstill was about 4 inches. It worked decently on lithium power, but I still considered it a fairly large and impractical vehicle during my ownership of it.
As for $4K+ bicycles, I personally agree, but everyone has their hobbies and passions where unreasonable amounts of money are spent, to the outside observer. I used to be really into mountain biking, but I still never owned a bicycle that cost more than $1K. I can't say from personal experience, having never owned such a high-end bicycle, but I imagine like most things there is a fairly early point of diminishing returns. I would say there is a vast improvement from $200 to $400, and still a significant improvement from $400 to $700, but it gets less noticeable as you go up. This is for things like smoothness of the drivetrain operation and such. These days I don't care half as much as I used to about bicycle specs. But yes once the price hits about $2K, I start thinking about scooters or motorcycles instead.
My current rides are a 20+ years old mountain bike retrofitted with a 48V 1000W front wheel drive kit, and more recently a moderately-spec'd department store mountain bike retrofitted with a 48V 1000W rear wheel drive kit. These are heavy (by bicycle standards) if I ever have to lift them, but they are perfectly rideable under pure pedal power and the weight is not terribly noticeable. Both of these have top speeds higher than what I feel safe doing on a bicycle, but the power is there for hill climbing and just a little throttle is great for casual cruising or running nearby errands, making them great practical machines. I know it's off topic, but I highly recommend putting together such a custom bike. It really doesn't cost much and they are great fun to ride, almost as much as scooters IMO.