It turns out that buses need effective interchanges to provide anything resembling useful connections. Those require a lot of space for the venue itself that typically is not available. They also require extremely high connectivity into and out of that venue, which is typically not how existing roads are configured.
It also turns out that buses block up normal-width two-lane roads every time they stop. The amount of interference between just the buses themselves grows far faster than the number of buses on the road, because it is very difficult for them to pass each other when one is at a stop. They similarly cause obstructions to other road users in large motorised vehicles, including delivery vehicles and emergency services. And they force smaller, less visible and more vulnerable road users like cyclists, motorcyclists and crossing pedestrians into relatively dangerous situations when passing or crossing near them.
Buses are efficient in terms of fuel/environmental considerations only in situations where there is a critical mass of passengers wanting to travel on the same route at the same time, which you do have in big cities like London and Paris, but don't in smaller ones. That leads to either having little or no service where there is less demand or running large vehicles that are actually less efficient than smaller private transport on a lot of routes outside of peak times.
Bus journeys are inevitably very slow compared to private transport, because of the circuitous routes, frequent stops, large and slow vehicles, and often some extra time walking to and from the stop at each end of the journey. A typical journey by bus can easily take 2-4x as long as it would by car and may be little faster than walking and much slower than cycling at congested times.
Buses lack flexibility, particularly for passengers carrying large amounts of baggage, unwieldy items like pushchairs, or themselves disabled and travelling with equipment like wheelchairs. Often there is not enough space on the bus if more than one or two people with extra things with them want to travel at once, and stories of people having to wait two or even three buses before they could get on are not unusual.
Buses are horribly unhygienic compared to any form of private transportation, particularly in colder months when often there is little ventilation and lots of passengers generously sharing all of their bugs with everyone else aboard.
Buses are less comfortable than almost any form of private transportation.
Basically, buses suck. They may be somewhat more space and fuel efficient under ideal conditions, which often do not exist, and what advantages they do have in those respects are often mitigated or cancelled out by their numerous downsides. Almost any imaginable transport system, public or private, from simple walking and cycling to high-tech underground metro systems, is better than buses. Buses are awful.
Paris is currently building more than 200km of subway (between now and 2030), a new suburban train line (RER E), 4 new tramway lines...
The reasons why existing public transport systems don't have enough capacity to serve everyone is that (1) under the dominant hypercapitalistic ideology, public transport is often seen as something that has to be economically profitable, or at least break even, and not as a right (as you can always take your car); and (2) it is hard for it to be economically profitable, or at least break even, when it has to compete with the convenience of the car (which is effectively subsidized), apart from competing for space.
If cars were banned, public transport would improve by leaps and bounds by simple market force. There would be much more demand and it would be much easier to make it profitable (and there would be a case to run it with taxpayer money even where it weren't profitable. With the savings on buying cars and gas, we could very well pay that).
In the 1920s, my city had about 80K people and there was a tram line to a town about 15 km away, which had 7K people. It was operated by a private company, and was profitable. It was removed when cars became popular. Now, my city has around 250K people, the other town has 15K people, more proportion of people work and thus need to commute, my country and area are much richer and prosperous than back then, but no one would even dream of building a tram line like that. It would be considered pharaonic, unprofitable, etc. Why something that was possible almost 100 years ago, with less resources and worse technology, cannot be done now? Because there are cars.
I do agree with you that it would require a radical change in the funding model for public transport to successfully replace cars with a much larger-scale version of existing public transport infrastructure, but as I noted before, that would probably be prohibitively difficult politically in most places today. I think there are much more realistic alternatives, as discussed in some of my other comments here, that we could work towards over perhaps a 10-20 year time frame instead of more like 50-100.
That's quite a generalisation. People in major cities like London and Paris would obviously like to clean up the pollution and reduce the noise and disruption, but the simple fact is that the existing public transport systems are not even close to having enough capacity to replace private motor transport entirely, and there zero chance of them having it any time soon. And those are the ideal cases, where there is enough demand to make a reasonably efficient and close-to-24/7 public transport network viable. Those in smaller cities have much worse options in terms of efficiency, availability and reliability, as do those travelling between or outside the bigger cities.
You can't just ban cars to fix this, or price them off the roads with congestion charging or the equivalent. It would take a huge programme of investment in developing new mass transit alternatives and redesigning large parts of the overall transport infrastructure within cities to make forcible elimination of cars in favour of public transport a viable option, and any such plan would probably have to be implemented over a 50-100 year timescale, with those footing much of the bill never living to see the benefits. That's a tough sell for any political administration.
I personally think that making private transport less intrusive and damaging has far more potential to improve the situation in the short to medium term. Infrastructure for electric vehicles is one obvious move in the right direction, but more generally, our road systems are still designed around the idea that one person making a journey with limited cargo needs a car-sized vehicle, which is horribly wasteful in road space, parking space, fuel consumption, etc. We have some very basic infrastructure in cities these days for bikes and we make some allowances for motorbikes, but these modes of transport have serious downsides for a lot of people. Crucially, the changes needed to improve those downsides would also allow, for example, smaller electric vehicles that are still fully enclosed and having space for carrying small loads in addition to their human occupant. Combine that with increasing automation over the next couple of decades for better safety and general efficiency, and most of us might at least still be alive to see the benefits.