The point about a big heavy thing and a smaller thing not being in the same space applies equally to busses / lorries and bikes in the same way it does with a bike and a person.
This is the reason that bikes were taken off the pavement in the first place, since having a piece of steel / plastic/ aluminium / carbon fibre inserted into you at 30mph can put a real downer on your day.
There' are a couple of small problems with your plan
- The footpaths are raised to prevent vehicles coming into that space
- many places have large bollards to stop lorries parking on the pavement
- There is always some hoarding for the current re-development that narrows it to half a person width
- Dropped kerbs that make it difficult to walk along the pavement
- The moron coming the other way with their head in their phone.
- The kamikaze cyclist who slows for nothing
I'd suggest that we connect EVERYONE up to some high voltage source where the ability to activate is controlled by near-field communications, that way the bike can zap the lorry / bus driver who is too close, the walker can do the same to the idiot on the bike that is screaming towards them or the person with their head up their mobile phone that is just about to walk into you.
The other obvious way to solve this is to just do away with the commute, remote working is easy, the technologies are there, so it would reduce the amount of traffic in all forms, thus reducing the problem.