Why Saudi Arabia Is Building a Linear City

I'm not even close to being a civil engineer, and I can point out so many fundamental flaws with this city, that only a complete mental defective, or an absolute monarch to whom nobody would dare question their edicts, would come up with.  Just a handful of items that could easily see the city fail:

1 - Since it's literally just a spur across the strait from an Egyptian city, it's fate is now inextricably bound to the fate of a city that it has no control over, in a completely separate country.  If that port/resort city fails, this Line could fail as well.

2 - The city is proposed to be based upon technology that, as far as I know, doesn't yet exist, such as high order AI.  If you base a city's existence upon technology you don't have in your hand, or which isn't available to purchase, then the survival of your city can easily be wrenched away if those technologies fail to be invented, or invented on a time scale that allows your plans to follow through.

3 - Like ALL planned cities, it's rigid form means that any expansion is not only going to be haphazard, but it'll end up ultimately defeating the purpose of the city in the first place.  If you do not design your city in such a way that expansion can occur in an organic and productive fashion, then it can easily spiral out of control.  This is even worse when you consider that the Line is basically so hard-built to be as-is that expansion is almost impossible, which means the inhabitants of the city could end up in a scenario not so dissimilar than to the Mouse Utopia experiments (look them up if you want to get a potentially horrifying glimpse into this city's future), wherein a city doesn't grow BIGGER, but DENSER.

4 - The city's not even a fucking line, not all the way across at least.  The further east you go, the more the mountains clearly interfere with the city at surface level, and the line becomes more or less a dotted line, or more realistically a series of island cities along the desert wasteland.  While I'm sure the lower levels are contiguous, this means that the construction would involve kilometers of blasting in the eastern regions to clear out all the rock it would take to build the lower levels, almost like a tunneling operation for a train like the Chunnel, but for significantly longer.  Given that there doesn't appear to be any rhyme or reason why the city is oriented that way, by the looks of the map, they could've easily given the Line a different orientation and avoided this problem entirely.

5 - With the eschewing of surface roads, and the clear reliance on trains to move the length of the city, the inhabitants are at the mercy of this singular artery of transport.  If anything happens to that train that renders it inoperable for any length of time, the city essentially grinds to a halt.  If the surface is entirely pedestrian, that means that it won't be constructed in such a way as to allow it to be used in any improvised fashion if the train line fails.  It is a fatal flaw to have your city's existence survive on a single artery of transit.

6 - The greenery of the city will need constant maintenance in order to not be lost to the desert.  Since it's so narrow, it's entirely exposed to the desert winds and the movement of sand, which over time will choke out the plants.  A normal city would be able to create a bulwark against this, and further expansion of the greenery across the desert might even be feasible.

7 - The whole project just screams art project, some neat idea; not a place that people would want to live long-term and build families and thus communities.  People might visit, but because the thing is artificial from the ground up and doesn't take advantage of the native topography, nobody would really want to stay, outside of perhaps a handful.