Well yeah it's not the case that one can add a single line to connect a completely new destination to an existing rail network and everything will scale perfect

Well yeah it's not the case that one can add a single line to connect a completely new destination to an existing rail network and everything will scale perfectly, but I didn't claim that. And as long as we're talking in abstract we can construct scenarios where one is faster or better. However my read the original comment was that that adding a new connection involves adding new infrastructure, which isn't always true. See the new European overnight routes that opened up post-Covid and which operate on the existing track