Having a quick look-see at Steam achievements, seems like about 40% of players completed the game (which is actually above average for game completion numbers, I think) and 35% beat Malenia. Funnily enough, that's more than those who beat Fortissax and Placidusax, both at 28%. Even Mogh the Omen is at 33%. Now all those bosses are out of the way so that helps explain the stats, as I doubt anyone would seriously argue Fortissax is harder than Malenia. He's just tucked in the deep end of nowhere, at the conclusion of a questline loads of people likely didn't follow.
For funsies I'm comparing it with BG3. 20% of players completed the game. 3.4% did it on Tactician, the hardest difficulty, which most CRPG veterans will argue isn't that hard at the end of the day. It's been out for a lot less long than Elden Ring of course but the disparity is still fairly significant. I'm not going to write an entire thesis based on those numbers, but it doesn't seem like Malenia is directly comparable to a hard mode.