You have the wrong metaphor, since bridge projects are all more or less the same, but software projects aren't.
I like the shed and skyskraper metaphor better. It makes a clear distinction that some projects are sheds (can be a single script) all the way up to skyscrapers.
Some projects don't even need to be extended afterwards, like most computer games.
It doesn't really work with either metaphor. Somebody building the Golden Gate bridge or skyscraper knows that they're building it and has a clear picture of what it will look like once it's finished.
Software is more like a settlement. It may start out looking like a village and end of life looking like a village or it may catch a wave and end up looking like a megalopolis - perhaps a clean, well organized one like Singapore or a disastrously organized one with chronic traffic, corruption and crime like Lagos.
A lot of people out there think that they need to make space for a subway in their village coz it's Definitely Coming One Day while others don't have time to build a subway even while they suffer 7 hour long traffic jams.
I like the shed and skyskraper metaphor better. It makes a clear distinction that some projects are sheds (can be a single script) all the way up to skyscrapers.
Some projects don't even need to be extended afterwards, like most computer games.