IMO, at any school with baseline prestige that's the same as the rest of the conference, having a consistently winning record in conference should be plenty. If you're coaching Austin Peay, making the PT every three years is unrealistic expectations. Should there still be firings at Peay? If you've been there 10 seasons and have an overall conference record of 64-96, with a peak of 9-7 in your 6th season, then yeah, I'd say you can be fired. But if you're flitting between 8-8 and 12-4 every year, winning the regular season crown every once in a while, that's enough to keep your job, even if you never end up in the postseason.
Whereas at Memphis, with a baseline prestige higher than anybody else in the conference, the standards should be higher. And at Virginia, with piles of recruiting money every year, they should be even higher than that.
Again, I'm not a D1 player. I understand that my impressions from real life may not always match up to HD. But I think that, once you've factored in that turnarounds in HD D1 take longer than in real life D1, real life D1 provides a good first approximation for appropriate standards.