There's no need to make anything up. Mass transit systems are relatively efficient if and only if they are used on routes popular enough to replace enough private vehicles to offset their greater size and operating costs (both physical and financial). That usually means big cities, or major routes in smaller cities at busier times.

Achieving 24/7 mass transit, available with reasonable frequency for journeys over both short and long distances, would certainly require everyone to live in big cities with very high population densities. Here in the UK, we only have a handful of cities with populations of over one million today. That is the sort of scale you're talking about for that sort of transportation system to be at all viable, although an order of magnitude larger would be more practical. All of those cities have long histories and relatively inefficient layouts, which would make it quite difficult to scale them up dramatically without causing other fundamental problems with infrastructure and logistics.

So, in order to solve the problem of providing viable mass transit for everyone to replace their personal vehicles, you would first need to build, starting from scratch or at least from much smaller urban areas, perhaps 20-30 new big cities to house a few tens of millions of people.

You would then need all of those people to move to those new cities. You'd be destroying all of their former communities in the process, of course, and for about 10,000,000 of them, they'd be giving up their entire rural way of life. Also, since no-one could live in rural areas any more, your farming had better be 100% automated, along with any other infrastructure or emergency facilities you need to support your mass transit away from the big cities.

The UK is currently in the middle of a housing crisis, with an acute lack of supply caused by decades of under-investment and failure to build anywhere close to enough new homes. Today, we're lucky if we build 200,000 per year, while the typical demand is for at least 300,000, which means the problem is getting worse every year. The difference between home-owners and those who are renting or otherwise living in supported accommodation is one of the defining inequalities of our generation, with all the tensions and social problems that follow.

But sure, we could get everyone off private transportation and onto mass transit. All we'd have to do is uproot about 3/4 of our population, destroy their communities and in many cases their whole way of life, build new houses at least an order of magnitude faster than we have managed for the last several decades, achieve total automation in our out-of-city farming and other infrastructure, replace infrastructure for an entire nation that has been centuries in development... and then build all these wonderful new mass transit systems, which would still almost inevitably be worse than private transportation in several fundamental ways.

Why so big though? I lived in a 25 000 people town in Sweden and did not need a car more than a few week ends per year. There was 5 bus lines for local transport, and long distance busses and trains with quite high frequency.

And that's not taking into account the fact that bicycle is a very viable way to move around in cities < 200 000 inhabitants.

I have actually never owned a car, I just rent some once in a while to go out somewhere where regular transports don't get me. I have lived in Sweden, France and Spain, in 10 cities from 25 000 to 12 million inhabitants. Never felt restricted. I actually feel much more restricted when I drive because I have to worry about parking, which is horrible in both Paris and Stockholm. Many people I know, even in rural Sweden or France, don't own a car because it is just super costly and the benefit is not worth it. It's very much a generation thing tough because my friends are mostly around 26-32 whereas nearly all the person I know over 35 owns a car, even if they don't actually have that much money and sometimes complain about it.

You've almost answered your own question, I think. Providing mass transit on popular routes at peak times is relatively easy. It's more difficult when you need to get someone from A to B that is 100 miles away, and then back again the same day. It's more difficult when you are getting someone from A to B at the start of the evening, but their shift finishes at 4am and then they need to get home again.

To provide a viable transport network, operating full-time with competitive journey times, without making a prohibitive financial loss or being environmentally unfriendly, you need a critical mass of people using each service you run. That generally means you need a high enough population density over a large enough urban area that almost all routes become "main routes" and almost all times become "busy times".