That really doean't sound like much fun to me, but we all have our different ideas of what's fun.
It's very much what you make of it. It's infuriating when you keep screwing up, but 90% of the time I can honestly say it's my fault. And while the annoyance is grand, so is the personal reward afterwards. If it were easy you wouldn't have the same feeling of accomplishment.
But I do believe, that at least in part, the fact that we find it so annoying to keep retrying and needing to actually improve our skills before overcoming the fight is due to expectations. Expectations created by all other games where victory usually comes fairly easy. If all games had this level of difficulty, it would just be the normal level of challenge.
I do have thoughts about when does playing turn into work, if you keep needing to run through the same scenario. I guess that's for everyone to find out for themselves. Personally, I can't remember the last time I had such a visceral and intimate relationship with a game. And there might likely never have been anything like this for me. A situation where the game itselves somehow becomes personified as this great adversary you wrestle with, have respect for, curse when you're fed up with it, but ultimately maybe try again.
But one thing is for sure, at least in my case. I don't think I would have had the patience to put up with it as much as I did (and I still have doubts if I'll actually finish it) had I not finished Bloodborne before this. That really was my entry drug into the house of pain. While Sekiro's gameplay is top-notch and better than Bloodborne's, I need more than just gameplay to get attached. And that's what Bloodborne provided for me.
It's also why I hope Elden Ring doesn't feature the same level of difficulty as Dark Souls or Sekiro. I have no desire to spend two hours on a boss in a game as lengthy as that.
It'll be interesting to see what approach they take. I'm particularly interested in the spin they will put on the same world, death and combat mechanics. And the story motivation for it all. But I do hope they incorporate as much of the combat systems of Sekiro as possible. The deflect/attack mechanic is too good to drop, imo. And just go back to rolling/dodging everywhere.