He can make a new character or the party can quest to have him restored some how (restoration is a spell no?). Actually, I think there might even be a sword +1 with wish in that module. It's all part of the fun.
Fighting a foe like Strahd is not something any group of PCs should take lightly. Such actions need to be carefully planned well in advance. How did they put themselves in that situation in the first place? What magical protections did they not have? Negative Plane Protection is only 3rd level and it should have been cast. Not to mention all the typical vampire slaying tactics that must be employed. My players would probably spend an entire session or two just planning an attack on Strahd.
When a game system doesn't allow the PCs to make use of preceptory magic and strategies against a foes weaknesses it becomes very dull and boring. I personally can't stand game systems that are strictly products of the mechanics. Which is why I do prefer 2e for D&D. It's encourages a different play style and it takes cunning and strategy on the part of the PCs to be successful. And yes, you have to embrace the suck sometimes, but that's all part of the game.
This is very interesting, but honestly, you should know what you are speaking of. Ravenloft came out for AD&D1, and there was no Negative Plane Protection then, since Unearthed Arcana was only published 2 years later. Moreover, what do you mean "should have been cast" ? How many characters in a party ? How many 3rd level slots does a cleric have ? How many hours in a day ? So how much of your party can you protect ? F*** all, that is what you can protect.
As for planning an attack on Stradh, most levels were lost before even finding him. There are level draining undeads all over the place, including random encounters like 1-4 vampires ! At level 5-7 ! Or 2d6 wraiths ! Or 2d8 Wights ! And this on top of all the encounters planned anyway. Come on, it's absolutely ridiculous.
And yes, there is a sword +1 with a wish, in a random crypt, and there are 40 crypts with random surprises in there like a banshee that forces the whole party to save vs. death, three hellhounds that burn to a crisp the characters in front, 1 vampire, 2 spectres, 1 ghost... And you can't even rest since Stradh is hunting you...
And the players planning an assault against Stradh... If you play him like a vegetable, yeah, right, have all their plans succeed, but it's not how he is supposed to be played either: "Strahd chooses when he attacks. Strahd is supposed to be a genius, play him as one. Whenever he is aware of the PCs' positions, he is allowed to make an attack how and where he wants. His attacks must be timed to be most advantageous to
him. To do that, Strahd must move around during the adventure. However, he can always be found in the place determined in Fortunes of Ravenloft. It is your responsibility to see that the vampire uses his abilities to his greatest advantage. 2. Strahd knows when to withdraw. He knows when he is in over his head. If he is losing a battle he becomes gaseous, polymorphs into a wolf or bat, and/or summons other creatures to guard his retreat."
After that, 2e is the worst edition ever in my eyes (for the game system, not the settings which were the best), a rehash of AD&D with zero added value and frankly no more strategy and no improvement on the play style. At least 3e, 4e and 5e tried really to change the game, and each contributed something that lasted, by I really struggle to remember any lasting contribution from 2e that made it in the later editions, except for the infamously unbalanced splat books that needed 3e to become usable.
I'm sure your players are fantastic tacticians, but I'd really like to see them trying the original Ravenloft with AD&D rules played by a DM who just applies the module. But I would settle for you really reading the posts before answering them in this way.