The relevant question, though, is whether things would be better, the same, or worse with AC cancellations or delays if there were no APPR, and one can't answer that by looking at what other airlines do, with different management, in different conditions, in different markets. Moreover the APPR isn't even properly enforced, so even if we could somehow compare AC performance today with AC performance in the counterfactual world where everything is the same except the APPR was never enacted, it wouldn't tell us what things would be like for AC performance if the APPR were properly enforced. (As an aside, OTP isn't that relevant a metric because it doesn't account for the length of delay, which really matters, as Cow in particular has pointed out before.)

Even if we ignore all of the above, it seems we shouldn't just compare AC with the US and conclude regulations are ineffective. We'd at least have to include EU261 in the comparison, because it's a much stricter and better enforced (and longer standing) regulatory regime. Otherwise we are just cherry picking the evidence base.

On my part I suspect there are just too many variables for a post facto analysis to be very illuminating. It would be too hard to isolate the effect of regulation.