In my opinion, Dallas and Phoenix aren't losing their chances to the Spurs this decade the way Reggie, Barkley, Ewing, etc. did in the 90's to Jordan. Those teams in the 90's were actual championship contenders. Dallas and Phoenix have never seriously been in the race for a real championship.
Dallas losing to Golden State wasn't a "choke" or "upset". Golden State was 3-0 vs. Dallas in the regular season. Dallas showed no signs of life the entire series, therefore didn't "choke" or get "upset" if they weren't in a position to win the series in the first place.
Over the last three years, Phoenix runs over all East teams and weak West teams in the regular season, gets 62 wins or so, but loses to top defense. They can go 78-4 vs. the rest of the league by run and gun, but against top defensive teams in a playoff setting, plain and simple they're just not a real contender. They were 62-20 going into the '05 playoffs, yet the Spurs nearly swept them, 4-1 in the WCF. Don't let high winning numbers fool you. It's about being able to score IN ADDITION TO playing some decent defense. Phoenix scores better than anyone, but probably plays the worst defense in the league.
Dallas entered the realm of supposedly "championship contender" on a Game 3 Duncan foul-out on a non-foul and no foul-call on Duncan's putback attempt at the end of Game 7, end of story. There's not much else to say about Dallas other than they're no better than any other West who has failed to win a championship. They are probably the league's luckiest team as far as getting bounces to go there way. Dallas losing to Miami in 2006 was not an upset, just a scenario playing out after the wrong team managed to get to the Finals. Phoenix, had they had Amare that year, probably would have won the championship depending on Miami's defense... Phoenix can get by Dallas in the playoffs, as in 2005, but not San Antonio.