Well buses and trains don't have seatbelts so it would be a bit difficult to fit them! When travelling on a coach, modern ones have special seats with car seats built in for 9 months + and they almost always provide booster seats even in the older models. We travel on a coach quite regularly with DS.
The taxi issue is because if you are stuck unexpectedly it's unlikely you'll have a car seat on you. In Australia you have to have one but taxis don't carry them (take up too much boot space) and I have heard of people being stranded because taxi drivers refuse to take them.
Baby in sling over seatbelt is better than baby in your arms but bear in mind that any object, including people, in a crash will effectively increase in mass meaning that they feel incredibly heavy. Slings are designed to carry/hold babies of normal baby weight perhaps up to a toddler and so they are unpredictable in a crash because it's quite likely that they will be dealing with a mass which is a huge increase of this. Even if the material holds them and doesn't rip, the stitching and buckles may not.
Plus the fact that the impact is forwards rather than down, they are designed to hold the baby in against gravity - you would only wear them when standing upright, even when you lean over most people's instinct leads them to hold the baby in. If you imagine dangling stomach-down off a cliff or a bridge, held by a harness, with the baby in a sling on your front, and then suddenly dropping and the harness stopping you (but it being a sudden stop) imagine that - it's about the closest you can get to the forces while easily being able to imagine it. You can also see the rationale for never having two people in the same seatbelt, especially when one person is a LOT heavier than the other.
You'd possibly be OK with a fabric sling in the first instance (most woven wrap slings are strong enough to be used as hammocks for multiple adults), but the second not necessarily unless their shoulders were also tucked in. Also bear in mind that if the baby's head and neck are not supported but the rest of their body is, this could cause the head to snap back suddenly which could presumably be fatal.