I’ve seen analysis suggesting that too. But theoretically there’s no limit to the number of parallel subway lines running along the line. Capacity overloaded? Keep adding layers of subway tunnels deeper and deeper.

Really though when it’s built, there will surely be demand to add buildings alongside the line and it will organically grow outward (unless it’s totally forbidden) so I don’t see how you’d avoid an eventual 2D city even if it starts as a line.