Heat lost in the first round in 2007 1 year after winning the title--but their opponent (Bulls) had 5 more wins than they did.
Mavericks haven't not lost in the first round since winning their title--but none of those teams were division winners.
Spurs also lost in the first round in 2008 as a 3 seed 1 year after winning a title, but that was to a 50-win Dallas team.
There are always differences from this year's scenario, but the point is that there are many examples of recent title winners losing in round 1. Of course it would be a longshot, of course it would be highly unlikely, of course if it happened it would be an exception to the rule; but that doesn't mean that the odds would be insurmountable, or that we should think that there's zero chance of pulling off the upset.
Yes one should learn from history, but one should not be a prisoner to it.