You made this comment twice, so I guess I'll reply to it twice, but the reality is that the acceptance rates of ATC was very, very low (again, under 13% in 2004). Low acceptance rates actually do indicate a shortage of good publishing venues (certainly relative to the number of submissions), but it would be interesting to look at the ATC acceptance rate over time; if it was much (much) higher in its final years, it would be easier to accept your assertion.
However, at least speaking as an academic, I wouldn't say that ~20% acceptance rate is necessarily indicative of a shortage of good venues.
There is plenty of not-so-good research that is submitted to top places that has no hope of getting in.
(My experience is from computer security research, where the acceptance rate of security conferences has gone down, but the fraction of good papers has also actually gone down, so the fraction of good papers getting in has roughly stayed the same.)
That being said, ATC seems to indeed have been a high-prestige conference back in the day, and hence indeed competitive.
My experience is from recent years, where it was viewed as a good-but-not-tier-1 conference in systems research.
