Insofar as depth of competition by decade goes, I can understand leaving out ppl like Ivanovic, Myskina, Jankovic, poor Safina in the 2000s. But in relative comparison to those players in their decade, Sabatini surely had far less impact on competition in the 80s (Jan 1st 1980 - Dec 31st 1989). No Navratilova, no Evert from 1985 onwards, how many Slams could conceivably have gone Sabatini's way?? In fact, where would those Slams Evert/Nav won have ended up?
Sabatini as a contender in the 80s is very borderline. You have the Three, and sort of a fourth.
Actually I think removing Evert and Navratilova would just show how shallow the 80s were. If forced to go through each this is where I think each would have ended up possibly
1980 French Open- Mandlikova
1980 Wimbledon- Goolagong (as was)
1980 U.S Open- Austin (or Mandlikova)
1980 Australian Open- Mandlikova (as was)
1981 French Open- Mandlikova (as was)
1981 Wimbledon- Mandlikova
1981 U.S Open- Austin (as was)
1981 Australian Open- Mandlikova (or Shriver or Turnbull)
1982 French Open- Jaeger
1982 Wimbledon- Billie Jean King (38 years old)
1982 U.S Open- Mandlikova
1983 French Open- Mandlikova
1983 Wimbledon- Jaeger
1983 U.S Open- Shriver
1983 Australian Open- Jo Durie (or Shriver)
1984 French Open- Mandlikova
1984 Wimbledon- Mandlikova (or Jordan)
1984 U.S Open- Turnbull (or Basset)
1985 French Open- 14 year old Sabatini, 15 year old Graf, or Kohde Kilsch (I am going to guess Graf)
1985 Wimbledon- Garrison
1985 U.S Open- Mandlikova (as was)
1985 Australian Open- Mandlikova
1986 French Open- Graf (or Mandlikova or 15 year old Sabatini)
1986 Wimbledon- Mandlikova (or Sukova)
1986 U.S Open- Graf
1987 Australian Open- Mandlikova (as was)
1987 French Open- Graf (as was)
1987 Wimbledon- Graf
1987 U.S Open- Graf
1988 Australian Open- Graf (as was)
1988 French Open- Graf (as was)
1988 Wimbledon- Graf (as was)
1988 U.S Open- Graf (as was)
1989 Australian Open- Graf (as was)
1989 French Open- Sanchez Vicario (as was)
1989 Wimbledon- Graf (as was)
1989 U.S Open- Graf (as well)
So by my estimation that is about 13 slams for Mandlikova who was very talented but a huge mental flake who often lost to nobodies in the early rounds of slams. A 15 year old probably would have won a slam at some point, be that Graf or Sabatini. Overall it is just a really weak group of players as a whole after Navratilova and Evert. Austin was finished at the top after 1981, playing out the final couple years of her career as a complete shadow of her old self. Sabatini and Graf did not really begin to emerge until later in the decade. Shriver, Sukova, Turnbull, Jaeger, Garisson, Maleeva, were all not especialy impressive, and Jaeger didnt last very long either. The rest like Hanika, Bunge, Kilsch, Potter, Jordan, Durie, Basset, Rinaldi, were very feeble excuses of even top 10 players.
I dont think Sabatini would have been helped much though. Graf had her number from Day 1 when they first started playing with the except of that stretch in and around 1991. She may have snuck out an additional slam but probably not because of Graf.
I did list Sabatini as a contender of the 80s though since she was the only person to beat Graf 3 times in 1988 and 1989. She did reach some big finals in semis. She was really the 3rd best player in the World behind Graf and Navratilova from 87-89 (arguably 2nd best in 1988 ).
I think if you removed Graf and Seles from the 90s or removed Serena and Venus or Henin (depending who you consider the 2nd best of the 2000s) from the 2000s the list of winners would still far eclipse what would have been seen in the early to mid 80s without Martina and Chris.