I think we are fine in term of infra -- I don't work as a civil engineer, but considering companies in my city repair the roads every year /s they probably retain the knowledge.
How easy is the machinery and tools needed to keep all the infrastructure running? Earth movers are pretty maintainable and barely depreciate. Survey tools though? They have gotten famcy right? Water management has gone quite high tech in some ways. I could see them falling apart kind of like when some hospitals had to revert to paper tape logs. It didn't scale anymore.
Exactly. Building roads and aqueducts were empire-level knowledge skills in Ancient Rome, performed by the army legions. Thankfully they're highly localized skills today.
