Software isn't bridges though. If you run into scaling issues if you're lucky it's just a case of swapping out a database for a bigger one. More likely it isn't just that though and being able to do things like scale up distributed workers coherently with a change of a config file requires upfront thought and design that YAGNI would say isn't necessary. People just breezily saying "we can optimise it later" for a scale up of orders of magnitude are nearly always wrong. I'd argue Reddit is a perfect example of a site that's clearly running into scaling issues but are locked into an architecture with few escape hatches built in

When a bridge fails it'll just fail. When software runs into a scaling limit it'll degrade and fall on its arse constantly and be absolutely terrible as long as it takes for the software team to completely rework their entire architecture often having to learn completely new technologies.