I think the burden goes both ways because Seles was 19 as of the Australian Open 93 win and well placed to keep getting better. For Graf, 19 was her earth shattering year. Even if we assume Seles peaked a little earlier than Graf, she would have still had another couple of years of peak performance left in her. So yes, the four slams Graf won between 93-94, those are the ones that would have come under attack. I do not think those matches would have been blow outs because both the RG and AO wins were hard fought. But I would favour Seles during that period because that's where their rivalry was at the point where she was taken out of the tour.The sample size is too small to make a judgement either way. The burden of proof is on those saying Seles would dominate Graf, when the h2h evidence just doesn't support this. Very hard to argue that Monica would not get at least 5 more slams however. I also wonder if Graf would have been pushed to improve her game & take the net more. It would have been a fascinating rivalry no doubt.
Said another way, in 2008, I wasn't looking at Fed's easy 2006 win over Nadal at Wimbledon but the much more competitive 2007 one. And Fed was distinctly playing below his 2007 level so we did fear a loss at Nadal that time, which is how it panned out. The fact that prior to the match Fed had a 2-0 H2H advantage over Nadal on grass had zero bearing on the result. Because Nadal was closing the gap with each year and Fed was getting older. That is roughly how I see the Graf-Seles rivalry panning out (that the effect of Graf wins over pre-prime Seles and in non-major meetings in 91 should be ignored) with two obvious differences - Seles had no such clear cut advantage on one surface as Nadal does on clay and secondly, the physical gap between the two would have started to close as Seles passed her early 20s, say in 96-97 because Graf was clearly the better athlete of the two. Fed enjoyed no such advantage over Nadal.