there's a lot of learning potential in building one or two but after that you're pretty much just making them bigger. There's a lot of engineering and construction involved but the new ones are really just very expensive and huge without netting necessarily much in terms of engineering science. In particular if you consider opportunity costs. With the 23 billion(!) it costs to build the new Large Hadron collider you could fund 230.000 100k grants to young scientists. That's enough money to fund entire disciplines.

And that's important to keep in mind because money is a limited resource and what's relevant is where that money didn't go rather than just speaking idealistically about the potential benefits of a megaproject.