Sustained profit can also be an indicator that one (or more) of the competitors has managed to differentiate their offering above the value level of the zero profit commodity suppliers.
True, but this can only be the explanation for one carrier--the differentiated one--usually a small/niche one.
Allegiant has made much better returns than the rest of the industry and, true to theory, they are profoundly different flying (and pricing) experiences. And Frontier and Spirit are trying to emulate.
And in little corners of the market we could defend the idea that competitively-isolated carriers would make better-than-standard profits (as JetBlue, Alaska, Virgin Atlantic did at the peak of their "upstartness") At this point Southwest is too big to sustain much of a claim differentiation, it just happens to hub at DAL, HOU, BWI, and MDW instead of DFW, IAH, IAD or ORD)
The current complaint is that the 4 majors (UA, AA, DL and WN), or the majors and their alliances UA/LHStar, AA/BA/OneWorld, DL/KL/AF/SkyTeam are all profiting--its near impossible that that's differentiation (if it were, we'd have to ask: different from whom?)
And satisfaction with air travel is generally down, so it is hard to make the case that the majors differentiated versus some other mode or leisure activity.
"Collusion" is the more likely than "differentiation" as the source of near-whole-industry profits.
{The other thing that undercuts any claim of "differentiation" is that we don't see any Major or any Transatlantic Alliance claiming a pricing premium. As discussed immediately upthread, the Majors/Alliances are practicing strict pricing parity. If they are all profiting, it is because it has been easy to maintain "the ranks" and none will "break ranks" and there's no fear of entry}