The first two and last bullet points might apply to motor vehicles, but the third and fourth certainly do not

They certainly do. It’s a fact that most cars and motorbikes are designed to be able to travel safely at speeds greater than our highway limits, and it can indeed be awkward to do low speed limits in a motor vehicle. For example my VTR runs faster than 20km/h at idle in first gear, which makes travelling in 20km/h zones annoying because I have to be on the clutch constantly. I could also claim that it’s unsafe to go that slow on a motorcycle due to instability, but really that’s just as true as saying 10km/h is too slow for stability on a bicycle - there is an element of truth in it but we both know it’s perfectly doable for anyone who’s been riding for a while.