The A380 completed the portfolio of an aircraft manufacturer that went from not even existing a few decades prior, to one of the top-two (and for some periods, the biggest) player in the civil aviation industry, now offering everything from small short-haul models to the biggest commercial passenger jet in aviation history.
So what happens after 2021, will the Airbus portfolio revert to incomplete? Airbus was already in the top two and for some periods, the biggest) player in the civil aviation industry. It didn't need the A380s 251 sales to achieve that status.
And from a technology standpoint it demonstrated Airbus's capability to build state-of-the-art, innovative aircraft, with technologies and refinements that subsequent programs such as the A350 are already benefiting from, and other programs will continue to do so.
The A300, A320, A330 and A340 demonstrated Airbus's capability to build state-of-the-art, innovative aircraft. Yes, the A350 benefited from technologies developed for the A380, but couldn't Airbus have got there without it? I think they could.
So yes, the bean counters will (rightly) say the A380 program in itself was not a success, but Airbus as a company would have been the poorer if the A380 had not gone ahead. Absolutely no doubt about that. The indirect benefits, now and in the future, cannot be quantified but are numerous, and immense.
If Airbus management knew in 2000 what they know now would they have gone forward with the program? If given a chance for a do over would they make the same decision? I doubt it.