Aren’t there a number of big cities that have been built by decree and work well? I’m thinking of capital cities like Canberra (AUS), Brasilia (BRA) and Washington DC (USA). Granted, these are not quite so ambitious as The Line, but they turned mostly uninhabited swampland and farmland into major cities.
I lived in DC for many years. There were people living there beforehand. It's at an intersection of 2 rivers. The original planning has been able to grow and adapt organically since then. But, go try to drive anywhere during rush hour (or anytime really) and ask again how well it works.
It's also worth noting that all three examples are capitals of their respective countries, which is a built-in reason for people to need to live there. The Line doesn't have that, so why are people going to upend their lives and move there?
It moved the capital far away from any population centre making the disconnection between central government and the people much larger. The city was designed with a vision of each city block providing all the basic amenities needed to live in the block: shops, office spaces, apartments, etc. This plan was scrapped by the military dictatorship ending with the city having pretty massive blocks where you need to travel long distances to do anything, it became a nightmare of traffic as it wasn't planned that people would require traveling all the time.
Like you mention, the only real reason there's a big population there is due to being the capital, all the bureaucracies of the State reside there and the cascade of employment created by that is basically its whole economy. As a Brazilian there is no reason to visit Brasilia except if you are dealing with the government (or feel like you need to see the capital's architecture for its own sake).
> Immediately after the end of the war, Canberra was criticised for resembling a village and its disorganised collection of buildings was deemed ugly.[96][97][98] Canberra was often derisively described as "several suburbs in search of a city Prime Minister Sir Robert Menzies regarded the state of the national capital as an embarrassment.[100] Over time his attitude changed from one of contempt to that of championing its development. He fired two ministers charged with the development of the city for poor performance. Menzies remained in office for over a decade and in that time the development of the capital sped up rapidly.[101][102] The population grew by more than 50 per cent in every five-year period from 1955 to 1975.[102] Several Government departments, together with public servants, were moved to Canberra from Melbourne following the war.[103] Government housing projects were undertaken to accommodate the city's growing population." [1]
[1] https://en.wikipedia.org/wiki/Canberra#Creation_of_the_natio...
Arguably, Canberra's local economy was/is able to rally around the workers who built the city followed by the influx of civil servants due to centralization of government branches and administrations. The same can be asserted about Washington DC or Brasilia.
In the same vain: towns that emerge around a single economic activity. Like College / University towns. Or mining towns.
Their common ground is a clear appealing, albeit single, economic activity that gives people the motivation to flock to the city. Canberra's local authorities only started to invest in housing projects after a clear need was asserted.
That doesn't mean those cities are regarded as "livable" or "favorable". Brasilia, by and large, is viewed as a "workcity" where most of the daytime population consists of commuters who leave at the end of the day.
Whereas The Line puts all the infrastructure in place without there being a growing community that needs catering to. It assumes that the appeal of the infrastructure in itself is enough for people to be willing to move in. I'd say that's premature optimization.