Airbus was a bit snake bit when it came to range. The A300/A310 was thoroughly outclassed by the 767, and originally the A330 was built to favor size over range (and therefore didn't sell very well) while the A340 had lousy performance, being somewhat underpowered in its first generation and then, of course, in its second generation it was embarrassingly inefficient relative to the 777-300ER. It's only been with the maturity of the A330 and A330NEO and with the A350 that Airbus has had a truly great long-range aircraft. The A380 was built with range in mind too, is the point.
However your comments regarding the A300 being outclassed by the 767 are disingenuous. The A300 entered service 8 years before the 767, so a bit like comparing the A300CEO with the 787. Had sales in the USA especially not been hampered by US OEM anti-competitive behaviour (and offsets and EXIM outside), ably assisted by US unions, more capable versions might have been developed.
The only mention that I make of the A300/310 was that it was outclassed by the 767. You provide some potential reasons why that was true, but don't appear to actually object to the fact of it, so I am not sure why you say that bare statement is "disingenuous". Yep, the 767 was designed later - sometimes there are advantages to being second. The 777 also came later than the A340 (and MD-11), and outclassed those aircraft too. Like I said, Airbus was a bit snake-bit relative to long-range widebodies.
In fact, Airbus did build more capable versions of the A300 - the A300-600 and the A310, which were developed at the same time as the 767. The A300/A310 eventually ended up with the same engines as on the 767 family, in fact, so to a large degree it was a straight up competition on the quality of the airframe and wing. Yes, Airbus had a decade-older design, but the fact of the matter is that these two families competed against each other for a couple of decades.
The A300 was designed for a market niche that in a deregulated environment really didn't exist. It was initially conceived of as a short-medium haul widebody. The A300's first revenue flight was Paris to London, and, even into the 1990s, Air France was still using the aircraft on that route. In the 1980/1990s I recall flying a number of shorter-haul intra-Asia routes on the A300.
If you look in 2019 at all Airbus and Boeing scheduled flights (globally) less than 2500 miles and divide them into narrowbody and widebody buckets by seats, you'll find widebodies account for only about 10% of all such seats (and obviously far fewer than that of flights). And most observers, hearing that stat, would say, "of course". This is now regarded as the natural preserve of the narrowbody. But it was the original A300 target market.
The 767 was also designed for that market niche, by the way. It was conceived as a US domestic aircraft. But the 767 had a lot of latent capability in its wing, far more so than the A300. So when it turned out that the US deregulated market was much less conducive to the 767, the 767 was able to stretch its legs and its fuselage to a far greater degree than the A300, becoming a truly intercontinental aircraft. Heck, the 767 is still in production, which is kind of amazing. It will probably still be flying 20 years from now, 60 years after first flight.
I happened in the 1990s one time to fly a 5000mi+ route where, within a week, in one direction I flew a 767-300ER and the other an A310-300. That route was at the limit of the A310-300's capabilities. Two class capacity on that aircraft was 220, as opposed to 260 for the 767-300ER. What I recall most vividly is that the A310-300's lav tanks backed up by the time we arrived (turning the lavs into no-go zones). The point being that in some fundamental ways it wasn't made for a flight that long.
You may or may not think the reasons for the 767 to have outclassed the A300 are "fair". But again, there's really no arguing with whether it's true and that's all I said.