Duncan was 1st Team All NBA in 13 and 3rd Team in 15 (and 2nd Team defense both years). He was objectively better than Wade. Parker was 2nd Team All NBA in 14. Kawhi was just entering his prime years (back to back dpoy in 15 and 16). Manu basically had a Bosh like career, though was definitely on the downswing at that point. They also had Diaw, Green, Splitter, Mills, even Bellinelli was competent. The 4th and 5th players in the Heat were literally in their last season in Allen and Lewis. Chalmers was starting and they had Cole, Birdman, and James Jones. Outside of Lebron vs Parker as best player, the Spurs were better at every single place in the rotation.
There is afterall a reason the Spurs were the betting favorite in Vegas despite the Heat having the best player in the world and being the 2 time defending champions. And there is a reason Lebron left his best friend after that season to go back to Cleveland to play for an owner that went scorched earth when he left and crazy Kyrie with Waiters, Thompson, and whatever they got for Wiggins with a first-year coach from Europe. Lebron knew what was readily apparent, i.e. Wade was done as a top level player and there was no way the Heat could build a contender with LeBron and Wade on the team. LeBron would keep them too good and they had no cap room or assets to trade. The well was dry in Miami.