The main criteria that all the experts use for considering Graf the GOAT is her feat of surface mastery of winning each Slam at least 4 times which IMO would never have happened if Seles hadn't been stabbed. Sure, Graf would most likely have won all the Wimbledons she wound up with (7), but as for the AO or the French...I doubt it. It's also been said on these boards that Graf's dominance was extended beyond it's normal life span by the absence of Seles and I would have to agree with that so although I give Steffi all due credit for still playing after the Seles incident and I consider her to be one of the very best, I just can't put her as the GOAT and I'm not one of those who is suggesting that Seles was a greater player overall to Graf because the way that things played out that just isn't true.
I do indeed put Navratilova just a hair ahead of Evert in regards to the GOAT discussion only because Evert wasn't able to win one of those Wimbledon or US Open finals against Navratilova in the 80s, but if she had done so she would be the GOAT in my eyes. In reality though, little separates any of the four best women (Evert, Navratilova, Graf and Court) and any of them have a legitimate claim to be considered GOAT. IMO, the GOAT list goes like this for me: Martina, Chris, Steffi and then Margaret Court.
Although I rate Graf #1 all time on my list, your reasoning for not is perfectly valid and I would not try and change your mind. I certainly dont expect people to unaminously pick Graf #1 just because I do, and while I try to be objective in my reasoning, I do expect to a possible personal bias, which is probably true of alot of people. I will give you a few of my own perspective on it is all:
1)Regarding had the stabbing never taken place there are many unknown variables of significance. Biggest of all is injury. One of Seles's benefits prior to the stabbing was having never suffered a significant injury. One cant really predict when players can suffer future injuries which hamper them though. Graf who is the much more natural athlete would suffer some significant injuries which affected her career as she got older. There is no saying Seles too would not have run into injury problem at some point. For example when you look at Seles's shoulder injuries in 1996, would you attribute that to the stabbing? Can you be sure those type of injuries, which are pretty much certain to impact her performance if they did take place, would not have happened without the stabbing?
There are also personal problems, and how a player is able to cope is uncertain. I am not one who is going to make excuses for all Graf's losses at a certain point of her career due to the distractions from her nutty father, but that is an example of a personal problem that arises for a player. While it should not still be an excuse for their unwillingess to commit to tennis 100% so many years later the Williams did suffer with their sisters death. Navratilova suffered for awhile with the emotional effect of defecting, then her relationship with Rita Mae in the early 80s when she went into a brief "slump". For Seles it would be something like her dad's serious illness and eventual death. Things like these are unfortunate but they can arise at any time, and how a player deals is hard to project, but it is also not hard to imagine it impacting their performance in someway.
2)Graf did not retire until after 1999 Wimbledon. I dont find it that inconceivable personally that she could win 2 Frenchs, 2 U.S Opens, 1 Australian Open, which is what she needed to reach 4 of each, given that she would have had 6 and half years to do it. It certainly would not require "dominance" at any of the events per say.
3)It is just my opinion but I am not sure the period of 1990-early 1993 was really Graf's best tennis. To reflect my point I could list the head to heads she had with all the top players not named Seles, and compare them to before that period and after. I dont usually like that kind of over-analysis of each players results, but it would give some possible strong statistical indication to what I felt while watching her play during that time too. Were Sanchez Vicario, an even older Navratilova, Novotna, Sabatini, all just coincidentally much better that time period then any other? Since all of those players fared much better vs Graf head to head during that time then they did before or after this period. Although in Sabatini's case I would agree that was the period of the very best tennis of her career, not so sure on the others though.
Again though I am not trying to alter your opinion, I just thought I would share some of my own thoughts so my reasoning is not out in the dark is all.