I think it was NOT a mistake to begin the program, but it was a mistake to finish it.
None of us have magic crystal balls that can reliably predict the future. Airbus took a gamble on a “super 747” and initially it looked like a great idea. The Russians were also developing something similar, though they were hamstrung by the fact that their economy was in the toilet after the USSR collapsed (though on hindsight, they are probably grateful that they didn’t go through).
Airlines and airports were positive about the airplane. Sure, it was going to be bigger than anything else, but many airports were willing to accommodate it. So, why not? As long as air travel was the same as before, the A380 would be a serious competitor to the 747.
What people didn’t realize was that hi-bypass turbofan engines matured. They give enormous amounts of thrust while minimizing fuel consumption. And they’re also deemed safe enough that you can fly long distances (across the ocean, for example) with just two engines—something unthinkable before.
This piece of technology pretty much killed the rationale for the 747 and A380. Why fly to hub airports like JFK, Dallas-Fort Worth, LAX, and Heathrow and take another flight to your destination when you can just fly straight to where you need to be?
At this point, it would have been wise to cancel the program or perhaps turn it into something else like a cargo freighter. But Airbus went with it and by the time it was launched, it was already on thin ice. If it was launched in the 80s or even 90s, the A380 might have enjoyed success.