You folks are so non-committal!
Of course we can't know, but why not speculate?
One thing you didn't include in your excellent piece, britbox, is that before the stabbing Seles had won seven of the last eight Slams she had participated in - a remarkable feat - including 2-1 vs. Graf at Slams during that span. I see no reason why she shouldn't have at least come close to continuing that pace; she was only 19 when she was stabbed, and while female tennis players seem to peak earlier than men - more like 18-24- she was on the early side of her peak.
That said, I don't see a ton of Slams taken away from Graf. Probably the '94 Australian Open, maybe 1-2 French Opens and 1-2 US Opens. That's a range of 3-5, in my book. So if we take off 4 and add them to Seles, Steffi still ends with 18, Seles 13. But that doesn't count other Slams she would have won due to sustained excellence, conditioning, and confidence.
So it is my view that Seles was, at her peak, one of the greatest female tennis players in history - equal to Graf, Serena, Navratilova, Court and Evert at their best. Actually, I might place her 6th behind those five, and only then because of her career from mid-93 on being relatively pedestrian. I think if she hadn't been stabbed she would have managed 15+ Slams and be in the conversation with the others for GOAT.
But in the end, we can never know, which is sad.