Hundred year old infrastructure is not the most sturdy, obviously this structure was past the end of its design life (leading to its failure).

Maintaining a structure doesn't prevent the concrete from spalling (as the chemical bonds that bind the concrete age and degrade), nor does it necessarily pick up on failing earthen tunnels in hard to inspect areas.

Many bridges across the country are going on 100 years old, and have a ~50 year design life. Structurally they are deficient, often undersized for the modes of traffic using them, and structural failure (which is very possible) would likely kill people. US infrastructure needs heavy investment, rather than the neglect it has seen for decades.

They are also the most competent and qualified to make that determination. It's the same logic with any expert, your plumber says your pipes need to be replaced because xyz, you don't believe them, they say it's your decision, pipe bursts floods your basement and takes your hot water tank with it. You call the plumber and have them do even more work and they take substantially more of your money. If they were actually out to make money then they'd wait until the infrastructure is literally dust at which point they can name their price and you'd have to eat it.

That is the same argument made to debunk climate change science. As if people dependent on a theory being true would set aside their morals and say that theory is true.