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.
No, software isn't bridges. When bridges fail, people die. When reddit crashes, people take a piss and pet the cat.
I don't know the relevant details about reddit but the assumption that the early reddit people could have easily built something more scalable yet there are tech reasons why the later people, with far more resources, can't.
As to the assumption that one knows the important bottlenecks 2-3 orders of magnitude in advance, that's just wrong.
"late answers are wrong answers" isn't just for real-time. That applies to products as well as signals.
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.