I'm sure there will be tennis warehouse users who would argue Safin > Medvedev even if Daniil wins 10 slams.
Hell there are a non-negligible contingent of people who say with absolute sincerity that he has the highest HC peak ever. to their credit, can you really dispute it after watching AO '05 or USO '00?
That being said Marat was cursed from the beginning, 1) with horrific injury luck for the most part and 2) loving the drugs, booze, women, and general fast life of rock&roll. Medvedev by contrast was married at 22 and seems to derive enjoyment from playing FIFA and squirting black ink at people at aquariums.
The way I look at it is: Medvedev already has Safin beat in consistency, but still has not reached #1 or won 3 Slams. So even with more consistency, he has yet to surpass Safin's achievements, which I think says a lot about their respective peak levels.
Others are much more interesting. Med has clearly better achievements than Davy (Slam champ, for one) and the obvious serve advantage he has makes him probably more effective overall, but Davydenko had serious fast court chops and imo was every bit as talented indoors as Medvedev though the results don't reflect it. Davy will ultimately end up 4th here.
Kafelnikov has the surface versatility that the other three lack (Davy and Safin showed flashes on clay but ultimately achieved very little on the surface, nothing on grass) while Kafelnikov was maybe the best player on clay for stretches of the 90s. Also bageled PETE on Philippe Chatrier in '96 on the way to a Slam which was epic. '99 AO was a weak draw but he did show a great level in '00 as well just ran into a great Agassi (who was probably even better than '21 Djokovic or '19 Nadal). Also a world #1. I think Kafelnikov is underrated overall & currently above Medvedev.
Rublev will not surpass any of these guys the way he's going, btw.