CapnZapp
Legend
I am a very experienced DM. I still need the monsters to be harder.The monsters have plenty of "oomph" when you have players that are playing characters "right out the box".
Where things break down is when the players know how to squeeze extra potency out of their characters, and you assume their DM doesn't know how to squeeze extra potency out of the monsters to match them - or, more foolishly, that the DM shouldn't have to do that even if they've enabled more potent characters than the norm through optional- or house-rules if they don't want the characters to be just flat-out more potent relative to the challenges they face than normally assumed by the game.
It makes sense to do what 5th edition has done and make the only assumption about player and DM skill that it will be roughly equal, so that a new DM and new players are on equal footing to sort out how to play, and experienced players can squeeze more juice out of the game but an experienced DM can handle that, and (hopefully) groups of mis-matched skill will cooperate rather than compete, so the more experienced members will help the less experienced members get their bearings rather than the more experienced side intentionally using that fact to keep the game skewed in their favor while others grow frustrated.
The game material has made efforts to help DMs do what is right for their group - but the truth is that their efforts aren't infallible. DMs will make mistakes, experience surprises because of those mistakes, and learn as they play on. Trashing the game and suggesting the DM can't also improve is just as unhelpful as would be (the thing that didn't actually happen in this thread) trashing the DM and suggesting the game couldn't also improve. So let's do something else, shall we? Let's talk about what the game could actually do better in specific terms:
What could Curse of Strahd say instead of, or in addition to, the paragraphs describing Strahd's tactics that would help a DM not have the kind of unexpected result and minor mistakes that the OP describes experiencing? How can the game teach a DM to do it "right" in such a way that no portion of the reason for things going "wrong" can be said to be the DM's completely understandable, entirely not condemning, human error?
Your acceptance of WotC's decision to make newbie players the baseline is baffling.
It lets WotC get away with the easiest and laziest possible monster design.
Not only does this defaitist attitude allow WotC to lower the bar significantly compared to previous editions (3rd and 4th), it also completely ignores the much better option to design for (at least) intermediate players, but perhaps designate one or two of the more tactical abilities as "advanced" to let newbie DMs (or DMs running games for newbie players) know they can safely be ignored.
After all, it is much easier to take away complexity than to add it.
If enough players are as understanding as you, I will have to design my own monsters if I want to even hope to entertain my players. And I don't want that. I want to be able to pay WotC money to do that for me.
The only way to achieve that is to raise awareness this edition has erred too far in the carebear direction. Make it known the monsters (especially at double-digit CR) are not up to the task.
You interfering each and every time is not a welcome intrusion.
You're diluting the message: We need more high-CR foes with tactical abilities that resist trivial takedown strategies. The simplification of monster ability lists has gone too far.
Monster design need to take the real DPR of high-revved heroes into account: like has been said above, if a level-appropriate hero is capable of 50 DPR, a Solo monster can't be given a mere 250 hit points. That just invites a one-round wipeout.