Sekiro is a great game, but was ultimately past the point of being worth it for me. Possibly the only action game ever made that made me feel that way. Played tough action games my entire life. I honestly can't imaging that 95% of gamers are fine with Sekiro as is. And that's okay with me too. I don't want him to change the game. It has its fans. I bet that completion rate is absolutely abysmal though.
For an open world game emphasizing exploration, I wouldn't think that was ideal though. Demon's Souls was seen as difficult at the time, but honestly it was more about observation and preparation than only roadblocking you into numerous bosses. Played more like a survival horror game than an action game half the time.
Sekiro doesn't even have a second weapon. There's just very little room to experiment. You pretty much do what is required, or leave. Even Ninja Gaiden has like a dozen weapons. Some players will click with one more than another, and more people can find ways to excel at the game.