The Eiffel Tower has come the farthest in it's lifespan and so I'll talk about it first. When it was first built, the people of Paris thought it was pretty damn ugly. They couldn't wait for the 20 year lease to be up so they could tear it down. It was originally built as a proof of concept and designed to be easily dismantled yet extraordinarily strong. Today the Eiffel Tower is relic of the building methods of the past, but it also the most visited, paid monument in the entire world. It is as much a part of France's identity as the city of Paris is, and it is a precious symbol of the French people.
The London Eye was also meant to be temporary, and serves little purpose. Just like the Eiffel Tower in it's younger days, the London Eye is seen by some to be an ugly landmark. The London Eye makes a lot of money, and has already surpassed the amount it cost to build it. Tourists from all over pay the fee to ride the giant Ferris wheel. In time the citizens of London may even come to accept the London Eye as a monument worth keeping around, and learn to appreciate it's beauty just as the French learned to love the Eiffel Tower.
The Falkirk Wheel has similarities to both these attractions. It is a proof of concept and serves little useful purpose. The locals also consider it a waste of money. It was marketed as a tourist attraction right from the start. I believe that learning from attractions like the London Eye, the people behind the Falkirk Wheel intended to bring in tourist money. However, unlike the Eiffel Tower and the London Eye, the Falkirk Wheel does not exist in a capital city that already draws a lot of tourists. The Falkirk Wheel may not grow to the extent the London Eye and Eiffel Tower have.
My conclusion is that other mega-projects have become tourist attractions in the post-industrial age, but designing a mega-project specifically to be a tourist attraction might not work. The Eiffel Tower and the London Eye are going through an evolutionary process, which for the Eiffel Tower ended with it becoming a massively lucrative attraction and national symbol. However without a prime location I fear that the Falkirk Wheel's evolutionary process may end prematurely.