I'm not saying that Miami can't beat the Spurs in a 7 game series; they can. But if you watch both teams play over the past month, there's no question who's been better; the team that hasn't lost.
You can compare any team on an 18-0 to every team in history and conclude that they are playing the best basketball of all time. The Heat won 21 of 22 games last season, the one loss coming by two points to the Mavs. They were playing great inspired basketball but didn't win it all.
The Rockets won 22 straight a few years back and didn't win it all. The Suns ran off 17 straight a few years back and didn't win it all. History is littered with those kinds of examples.
The Heat, Thunder, Bulls and Spurs were all vying for the best record coming into the final couple weeks of the regular season. The Spurs were 46-16, Heat were 45-17, Thunder were 45-17, and the Bulls were 47-15. Seems pretty even, right?
The Spurs and Heat packed it in and rested the starters, the Spurs won all four games, the Heat lost 3 of 4. Neither side cared as rest was more important than seeding to them. The Thunder and Bulls played their guys big minutes and chose to chase seeding and still lost a game or two.
Does that change the way you look at the 18 game streak? It should.