Well not in DMC5 it's not, that's for sure.
I play and enjoy lots of character action games, and grades certainly aren't the be-all and end-all of the experience. I'll strive towards not getting shit grades, sure, but I won't worry too much if I don't 100% SSS/PP the whole game. If a particular encounter ranks me down for not having a high enough combo, I'm not going to spend time figuring out what it wants. I'll just move on.
The real thrill of character action for me comes from efficiently utilising available tools, and overcoming difficult enemies quickly, and coming through unscathed- not necessarily memorising endless superfluous pretty moves to jam into a jump cancel exhibition for a grade. DMC5 simply doesn't scratch my itch. DMC1 is still basically the perfect character action game- enemies are fast and ferocious, but through skill, knowledge, and practice, the player can decimate enemies like they're nothing. It's the ultimate glass cannon experience.
With each DMC title, they've flirted with style/combos and placed increasing importance on it, but with DMC5 they go too far. In lieu of any real challenge until DMD, the combo experience is all it has. And unless you know about the boss trick, you've gotta go through a very easy game twice to get there. No major character action game fails like this (but as pointed out by another GAFfer, it's an inevitable consequence of a AAA DMC game in 2019 combined with Capcom's nervousness about staying true to the series).
And that's why it's boring.
I completely agree that the series changed to what you describe. In DMC1 the challenge WAS getting through the game. But it still had Ranks, and if you did well, that dramatically boosted your chances of getting to the end by giving you more resources to spend on life, skills or items.
I also agree the harder difficulties should be selectable at the start, they just chose Urizen as a weird wall for that. If someone wants to get fucked up on a hard mode, then let them.
Now what I really disagree with, is that the turn to increase combat diversity and creativity is what makes the game boring. I can understand being disappointed because of the lack of challenge but boring?
Even in my first playthrough as an avid DMC player, I found the game a bit on the easy side, but not boring. That thought never entered my mind while I was figuring out what attacks I could chain together, what I could cncel and how to do it all without eating shit.
I disliked the V sections at first too, but the more you play with him and upgrade his skills, the more you learn how to chain combos together in a safe way.
None of that is mandatory of course which is, I guess, OPs big problem with the game but it is still encouraged heavily.
If you just reject that encouragement and hammer Triangle all through the game the yeah, maybe that might be boring. But you would have noone but yourself to blame.
Or maybe it's just the difference between pragmatic and creative players, I don't know.
