For sure if it was the Heat the Spurs would have had a tough series. Zo would d-up Duncan or Robinson well, and Hardaway would have all over Avery Johnson. Duncan may have had to play even better and fully cement himself has the best bigman besides Shaq in the league all the way back in 99 if that happened, just to win his first ring.
playoffs are all about matchups.
https://www.basketball-reference.com.../SAS/1999.html
https://www.basketball-reference.com.../MIA/1999.html
Spurs would have played:
Robinson
Duncan
Elliott
Elie
Johnson
bench:
Steve Kerr
Jaren Jackson
Malik Rose
vs the Heat:
Mourning
PJ Brown
Jamal Mashburn
Dan Majerle
Tim Hardaway
bench:
Terry Porter
Clarence Weatherspoon
Terry Mills
Voshon Lenard
So the matchup advantages:
Mourning > Robinson (late career DRob vs. Prime Zo)
Duncan > PJ Brown (think of PJ Brown as the '90s Kurt Thomas)
Jamal Mashburn > Mario Elie (Mashburn was a good 3 at this point, Elie was clutch and hustled, but older)
Sean Elliott > Dan Majerle (late career Thunder Dan)
Tim Hardaway > Avery Johnson
Bench: push (Terry Porter/Voshon Lenard shooting ~= Jaren Jackson/Steve Kerr shooting, Weatherspoon ~= Malik Rose)
So the series would have likely come down to late defensive plays on the interior, rebounding (think Zo vs. Duncan) and home court advantage. Heat went 33-17 that year and Spurs went 37-13, so Spurs maybe in 7.