There are libraries and libraries. I think they are a wonderful institution and there is something romantic about the idea of them, and being in them; naturally that isn't always the reality.
Nosing into a book in a quiet corner of a grand city building is not quite the same as rifling through the daily papers in a brightly lit communal area of a dilapidated town centre building with the local bored/lonely/bookish. Each have their different pleasures and vibe.
Then there are the researchers, official or otherwise - e.g. see the teeming hordes in the British Library in London bagging their daily laptop spot.
As more and more gets digitised (which modern libraries are fundamentally core to and involved with) and the physical environments (formerly the intellect-grooming dens of the working-class) are seen as an irrelevance to be defunded - or merged with communal zumba classes and the like - with librarians replaced by floorwalkers and customer service we may see them decline, but it will be a sad day when there are none left.
Yes...now that I have kids, the libraries have great kids activities and I have gone back to the library more than when I had before kids. They also have educational adult activities as well like computer skills. They also have 3D printing, robotics, programming classes, etc so have kept up with the times.
To me it's like a community center with books, rather than a library.
The Washington, DC, libraries are not great, but they generally had plenty of people when I looked into them. In the suburbs, the Montgomery County system had excellent holdings, and seemed to do good business when I lived there. That was a while ago, though.
It probably differs by community and country but in Ireland at least I can say that public libraries are quite busy and provide a popular service.
Some recent stats for Ireland: https://www.gov.ie/en/policy-information/7468fd-public-libra...
And a look at the international picture: https://librarymap.ifla.org/