Yup, the payoffs of scientific endeavor are sometimes decades into the future. My biggest issue with these extremely big projects are having all the eggs in one basket in case it gets cancelled a few billion into the project (USA's SSC as an example). That money could have some to a *lot* of other science. Same thing here and also the ever-delayed James Webb telescope. I love the Hubble and was very excited for the JW but now it's more money, more money, more delays. IMO a bunch of smaller space telescope projects would have spread the failure and delay risk.
Unfortunately, scientists really cannot control what congress does. Congress cancelled the SSC as I understand it. Essentially, though, the LHC is what the SSC became.
All cutting-edge programs like this, no matter their scale, will almost certainly undergo delays and cost-overruns simply because the technology has to be invented for at least some of them. IMO, it should be expected. One can plan for those kind of things, but the unexpected almost always comes up. There are excellent methods out there for project estimation, but even those that are the most successful will not always be accurate.
I am a software engineer by trade, and my personal goal when I design software is to deliver a quality product. I can give you an estimate of how long it will take to do something, however, it is an estimate only.
From my experience, those that are not knowledgeable in software engineering tend to want things delivered on the estimated date no matter what. I once worked for a company that did a big hardware/software project. A decision was made by the company president to deliver both hardware and software that were by no means ready to go out the door.
In essence, the company shipped an engineer in the box with the product and it took that engineer three months in the field to get the product working. There was another company overlooking the effort with the possibility of a much larger contract. Because the company shipped a crap product that took three months to get working in the field, the company lost the other project. IMO, if the company had said that the product was not ready to ship and had delayed it until it was really ready to be shipped, the company might have gotten the additional contract.
The company president did what he thought was right, and then he include in the company's annual report a statement which, to me, translated to: "We may ship sh!t to you, but we will get it working eventually." Needless to say, it was not all that long until that particular company president found himself looking for work elsewhere because the company's BOD fired him.
It is well-known that shipping a product that is the best possible when it goes out the door costs exponentially less than shipping crap and then trying to fix it.
Apollo 11's moon landing was delayed by a year, but it happened. There is an excellent documentary series out there called "Moon Machines" that provides some details about the effort. Watching it gave me an appreciation for the immense complexity of the project and the efforts that it took to make it happen. There were 500,000 people working on the project, and pretty much everything had to be invented to make it happen. I am not so sure that it would have been as successful as quickly if it had been scaled back to smaller steps. In some respects, the Mercury and Gemini programs were those smaller steps.
One might argue that the JW space telescope is a waste of time and money given projects like the E-ELT that is now under construction -
https://www.eso.org/sci/facilities/eelt/ which will surpass Hubble by far in many ways. However, getting a telescope into space opens opportunities that ground-based telescopes are not able to offer. That is something you have to get right the first time; Hubble was lucky engineers were able to find a work-around the problem - and all the while - there was a perfect mirror for Hubble in a warehouse at Eastman Kodak Company.
Project hardware is designed to the needs of the goals of the project. In this case, AFAIK, no other collider technology would be able to meet the energy levels that the goals require.