I don't think that is a good excuse, nothing in this inventory system or menu system in general seems to be designed with the purpose of being fast. Sort of like a Fallout or Skyrim inventory system if I should compare it to something. Its not optimal, but functional enough for a single player game experience.Anyway, to answer op's question, you can't access inventory just by pressing "I" because inventory is designed to be accessed with 2 button input (3 buttons for weapon change). This is a PvP game, too, and changing weapons on the fly is a skill. Retaining control of your character while navigating inventory is a part of gameplay. So accessing inventory with one button would be unfair competitively.
Besides that, if I should guess the majority of people playing these games do it for the single player experience, so rather than designing the inventory system based on pvp, if that is indeed the case, I think they could solve that better other ways.
I just think its sad, because again these types of games where you control your character in 3rd person, is not new to PC and other companies have already designed controls for them that are so much better than Elden ring, so they could just have copy/pasted those and then make one that works for consoles.I haven't tried mouse/keyboard, but I can imagine that it's pretty clunky without a lot of practice. Most things about the game seem to be designed around the notion of controller-based inputs. There are big annoyances even about the controller though. I play only off-line, so I have no hesitance about using mods for the game. The most essential one for me simply stops the camera from resetting when I click the targeting button without the game being able to identify a target, because I'm at the wrong angle or something. Getting killed due to disorientation from the sudden camera reset got old quickly.
And a lot of these things are merely the ability to map keys probably. Like buttons to use healing, mana. And then you could map other useable items to other keys. For instance being able to use mouse wheel to scroll back forth between items, you can only scroll one way, because the mapping for going backwards is not there. The functionality is clearly in the game as you can scroll forward using the mouse wheel, it shouldn't be all that difficult to make it go the other way on PC.
In my opinion, we are talking so few changes to make the game more enjoyable on PC, even if they left most of the clunky UI, these few extra key mappings would completely change the game experience on PC.
Another thing they should add, is when holding down right mouse button when using a shield or want to parry and walk away from an enemy, your character doesn't turn their back to them like an idiot, but instead they walk backwards holding up their shield.
When using the crossbow or casting a spell, spawn a little targeting crosshair instead and have your character shoot in that direction, like a freaking normal PC game, instead of having to constantly turn on/off lock on target.
To me these things would make a huge difference in regards to the quality of the game experience.