I do rewatch plenty of matches just because my evaluation criteria can change throughout time and because it's nice to compare what the intensity of play is like between matches. And also the current tennis sucks so I gotta watch something.
I think most at the time did think Federer would win, the aura around him was the highest it's ever been (and ever was), not even close (as after that, Nadal was a permanent thorn in his side). He was considered truly unbeatable at the time. He lost two sets in 22 matches dating back to the USO QF, one to ScricaGOAT on home soil and one where he was in IDGAF mode against Moya. People were frequently calling him the best player to ever play at 4 slams (this is a true indicator of Federer's greatness, he was already getting called the most skilled/greatest talent ever at USO 2004). That's not true of anyone else, but with Federer people had seen enough already by that point.
While everyone knew Safin was one of the few guys who had a shot of unsettling him, no one expected Safin to be consistent enough to maintain that high a level over 5 sets (even though he made mistakes in the sets he lost, they still took significant effort from Federer) and actually beat him. In general this match is one of the few perfect storms you will ever see in tennis which is why I never get tired of it. Two of the 5 or 10 best overall talents to ever play at their absolute peaks and both actually are up to the task with no major dips over 5 sets. Easily 2 of the 3 most potent ballstrikers to ever play. Plus it's on a neutral surface. Given that, probably the greatest display of all court skill and talent ever seen. Every single aspect of the game (serve, return, groundstrokes, movement, net play, variety) was at a great to GOAT level from both guys. I don't how many other matches can even tick 3 or 4 of those boxes much less all of them.
To expand on this:
Serve - great from both guys
Groundies - Fed great BH (his BH actually was pretty damn good here, a relative weakness against Safin but he was hitting it very well, much better than at 05 USO) and GOAT FH, Safin vice versa.
Ball striking - Both guys GOAT level at early and clean striking off both sides along with AA.
Return - Fed GOAT level first, good to great 2nd, vice versa from Safin.
Movement - Fed GOAT level, Safin great, GOAT for his height. Both guys GOAT level at offensive footwork.
Net play - flat out great from both guys, both used it has a significant strategy, Fed employed quite a bit of true S&V, Safin used S&V more on sure things, both guys were very solid on routine volleys and dug out plenty of tough ones.
Variety - GOAT level from Federer (judiciously mixed up power baselining with rope a dope, serve and volley, chip charge, angles off both wings, even droppers, much of this on the biggest points to boot, truly one of the great displays of variety ever honestly) great from Safin at least by modern standards(did plenty of damage with BH angles, approached the net plenty himself).
Shotmaking - nuff said. The humongous frozen rope BHs, Fed with the 103 mph FH, the droppas, drop volleys, the flick half volley BH lob, cold cocked returns
I really can't say this to be true for any other match really. Either the players themselves were lacking in a skill, the skill isn't as featured on the surface, or wasn't that good at it on the particular day. The other best matches I can think of would fall short of this two way level in at least 2 areas, and many of them 3 or 4. And I always say that the first 4 sets if Fed had won would have on themselves had a strong case with the other all time 4 setters (you can easily put it above 11 RG, 04 Wimby, and it's right there with 01 USO). On top of that you add maybe the highest quality fifth set ever with intense drama.
