There was never really any differences from DX9 to DX10 visually to begin with. There were cases like Lost Planet where DX10 had features locked into DX10 mode only, but were easily able to be done on DX9 as well. Fur for example could be rendered on DX9 just fine, but LP had it for only DX10.
(EDIT: It does seem I did say extra visuals when I said RE5. Sorry about that)
Your argument doesn't change the fact that DX10 was available, and on several different video cards - DX10 makes a difference in performance for the better (increased FPS) which is what the API was originally intended for. Not visual fidelity.
And RE5 is not an average looking PC game.
Not the best video in the world, Youtube can be so damn shoddy with compression (could be the user's fault as well), but it does good work.
The cutscene alone is filled with detail no other game - on PC, 360, or PS3 - really can put off aside from the typical Crysis argument. You can see the tiniest bits of details down to the textures in each character's clothes. You can see the patch details in the BSAA emblem on Chris' shirt or even the etchings on all of his belts and clips. Then the character's faces are amazingly detailed too. It's on par with Crysis's head model detail. You can see the pours in Chris and Sheva's faces, and the veins in their eyes even.
Then there are the physics of everything. Wesker's cloth jacket is very realistically rendered and even has incredible details just like Chris. Both in the cutscene and gameplay too. Oh and the guns as well. Each and every gun has immense details to them. Particularly the default pistols Sheva and Chris wield as shown in that cutscene. They have insane tiny details on them in the models, and so many moving parts to them that it's a total sell on believability each time they fire the gun. It's like Isaac's RIG in Dead Space 2. Not to mention there isn't any video game cutscene magic here, it's all ingame rendered right infront of you with the same assets.
The environment itself, particularly the ancient underground compound their in is particularly detailed again both in the cutscene and gameplay. The lighting on the walls and the entire room alone is pretty awesome, not to mention the details of the wall textures and such. Not to forget what the lighting does on the characters. Shadows are constantly being done on the characters models and the environment all the time. That cutscene alone has alot of different lighting effects on the characters that are really eye pleasing and realistic as they move. During gameplay, there are quite a few amazing smoke/light effects particularly during the Licker compound and the test subject chambers where spinning fans casts shadows. The expansion to RE5 - Lost in Shadows - capitilzed on this lighting effect with the mansion from RE1. Lightning casts shadows throughout hallways that are incredibly amazing. Especially when the enemy's shadow is on your screen, and you can't see him (CLASSIC!
) Enemies cast shadows too, and each enemies gets their own realistic shadows that are just as good as the main characters.
There isn't any feeling of "repeated levels" in RE5 on a whole. While a bit linear, ever chapter of RE5 has an entirely different layout and feel to it as well as enemies. Enemies have multiple model variations so you're not always shooting the same guy over and over, and details in them that are very close to the details in the main character models.
Don't get me started on the animation and cinematography of the entire game and cutscenes particularly. I could write a damn essay about how fantastically done each and every scene it done. If I recall correctly from the RE5 art book I read, the mocaps did every tiny ingame animation over at least 30-50+ times and all the scenes dozens of times over as well. Not to mention the director's fantastic camera and set work.
RE5 doesn't have too many graphical "set-piece" scenes like a benchmark would, but on a whole RE5's incredible attention to detail particularly in the textures is incredible. And the best part is, you can run the game with 100FPS+ no sweat. DX10 even offers improvements to frame rate with some video cards, like it's actually intended. Most ports, yet alone actual PC games, don't offer this option, yet you want to argue about it's inclusion.
RE5 is an incredible visual powerhouse you completely misjudge and underwhelm. Most of the games on the PC don't have half the detail that RE5 has (yet alone work properly), and are always focusing on giant WOW scenes meanwhile the rest of the game is dull and recycled stuff. Metro 2033 is an amazing looking game, but most of it is just the same stuff you see from the beginning of the game with really fancy smoke lights here and there.
The MT Framework engine is an incredible force to be reckoned with. Even on the consoles themselves, alot of the amazing visual elements of RE5 that make the game look so great can be found on them..