No. Boston is not a contender because its top 2 players do not compare favorably with the top 2 players on the actual contenders. You win with your best players not your bench. A great bench helps immensely in the regular season and can help turn a tight series, but at the end of the day if your best players aren't good enough then your team is not good enough.
Good narrative, but not true.
The Spurs top two players at that point were not on the same level as the Heat. Same with the Pistons. The first year the Warriors won they weren't considered on the same level as other talent they faced. Even the Mavericks didn't have two top players comparable to those they faced. They only had Dirk, and at the time, he wasn't considered on the same level as the top guys.
There are plenty of examples where what you said just isn't true, or the narrative shifts after the championship to say that the winners had true superstars all along, and now that's how they are remembered.
Curry won the MVP the first year the Warriors won the title. You don't need to shift narratives after the fact, when before the fact the media said he had the best season in the sport. And that year, Curry's Warriors defeated the Pelicans, Grizzlies, and Rockets before beating the Cavs. Curry was clearly the best player in every series before Lebron. As for the Cavs, the 2nd best player in the Finals was probably Tristan Thompson (I'm not joking, if it wasn't him it was JR Smith). So sure, Lebron was the best player in the series, but Curry was 2nd, Thompson was 3rd, Green was 4th, and Iguodala was 5th. Heck Barnes might have been the 6th best player in the series. So yeah the Warriors with the 2nd and 3rd best players in the series (including the league MVP) compared favorably at the top to the Cavs. Even if Irving and Love were in the series, they would have compared favorably. It wouldn't have been the same running down the list, but either way Curry was always going to be the 2nd best player to Lebron and they always would have compared favorably.
As for the Spurs, Parker was 2nd Team All NBA and both he and Duncan finished tied for 12th in MVP voting. Had Duncan not missed a bunch of games he would have been on an All NBA Team as well and would have finished higher in MVP voting (he was 1st Team the year before and 3rd Team the year after and was 7th and 10th in MVP voting those 2 seasons). Kawhi was still rising but was a 2nd Team All Defense and finished 11th in DPOY voting, so his defense was starting to be a known commodity. Obviously Lebron was the best player in that series, but Wade was no where near prime-Wade at that point (54 games and just 19, 4.5, 4.7 stats). In fact Wade's last appearance on an All NBA Team was the season prior to that one when he was on the 3rd Team (part of the reason the Heat won that series was Wade was still a top tier player, he wasn't in 14). Bosh was always an after-thought on those Heat teams. So the Spurs had the 2nd and 3rd best player in that series, which is comparing favorably at the top.
Obviously the Mavs was a big upset, but Dirk was also clearly the 2nd best player in the series. Dirk was absolutely amazing in his prime and he was full on prime that season. Now obviously Tyson Chandler was not on Wade's level (though he did finish 3rd in DPOY), but there is a reason that series is considered one of the biggest upsets in Finals history. Exceptions don't make the rule.