I think the argument isn't necessarily that no operation of that size could be kept secret, but rather one which involves killing thousands of innocent people would almost certainly have people deciding not to keep the thing a secret.

With something like this, it's likely that everybody involved saw their participation as a good thing, and saw no reason to break secrecy.

You're talking about two very different things during two very different times.

9/11 was an event witnessed by the public on a massive scale. This was a private development only witnessed by those who needed to know.

You didn't have the internet in 1950s, so even if somebody did know about it and told their friend or grandchild, would that person have found it interesting enough to tell more people.

Lastly, is the attempt to make a UFO really a conspiracy?? It didn't work, so there is very little to report.

There's no indication that they did make it (at least at a level above wind tunnel mules), and at least a little bit of data suggesting that they didn't. Avro Canada was stuck on two visions at the time: the saucer concept and the Arrow. Toward the end of Avro Canada there were concept illustrations of a Mark III Arrow, which had a "saucer section" embedded in an oversized Arrow fuselage in place of conventional (delta) wings. (The illustrations lived on into the '60s, if I remember my "How and Why Wonder Books" correctly, since they didn't fall under the "destroy all Arrow data" decree, being substantially dissimilar to the CF-105.) That would seem to indicate that there were terminal stability/control issues requiring at least a rudder and conventional directed thrust, given the technology of the day -- but with, perhaps, enough promise that there was some genuine fear that the Soviets could make something of it. The MiG 15 had come of something of a shock not long before, and Sputnik was, I think, foreseeable by then. Putting the 50-year stamp on the project may have seemed very reasonable at the time, even if nothing viable came of it.

They didn't keep it secret for 50 years. It even has a wikipedia page: http://en.wikipedia.org/wiki/Avro_Canada_VZ-9_Avrocar

That appears to be a different craft.

We don't know how secret this was? Did the KGB / other spy orgs know? It's possible many UFO saucer sightings were test flights of this or similar craft. Also, I've heard of the Avro saucer car before.

In a time before the Internet, and around the cold war. Both very important factors with cascading effects and repercussions.

An another explanation for those sorts of conspiracies (9/11, JFK...) is that the conspirators were not the perpretrators. Instead, they were people who 'left the door open' and allowed an attack that otherwise would have been easily prevented by the security services. That would require far fewer people.