Thanks for the encounter report, Celtavian. It sounds like WotC almost pulled a Bruce Cordell (Heart of Nightfang Spire) on you, there.
It does also look like a primer for handling high-level challenges, and shows 5e delivering on some of it's promises:
It does, for instance, seem reminiscent of the classic game, as 5e has tried so hard to be. You have a high-level monster that's highly resistant to spells, and needs magic weapons to hit. You have the fighter wailing on it with a magic weapon meant specifically for it. All very familiar in concept, if a little different on the details. Admittedly, the casters backing off and buffing is a little more 3e than classic, but then 5e is comfortably situated between 2e & 3e in a lot of ways.
Pleasure to provide it. The wizard was a big concern of mine with the new edition as was magic in general. I needed magic to feel like magic meaning different than the capabilities of martials, more powerful than what other classes (especially martials) could accomplish (not necessarily in terms of damage, but in effect), and tactically interesting in terms of problem solving in and out of combat. .. At every level you pulled off things no one else could pull off...
It sounds like you were happy with the wizard's magic 'feeling like magic' and being 'more powerful' and tactically interesting, for 15 levels...
I imagine the martials had fun swinging. It was all over fairly quick. 500 plus hit points gets eaten up pretty quick by a Great Weapon using Fighter/Barbarian with a magic dragon slaying sword and a smiting paladin using Vow of Enmity.
They must have done, if they were able to have fun with it for all of the preceding 15 levels...
Not a very fun encounter for a caster.
The wizard is pretty powerful when the deck isn't stacked against him by designers that make opponents with flashing neon signs that say "Only martials may kill this. Buff them and read a book."
Such are the vagaries of artitrary spotlight balance.
So the three "martials" (one barbarian/fighter, one divine half-caster Paladin, and one arcane full-caster Bard are all 'martial,' in your book, it seems - maybe you meant 'melee' or 'doesn't completely disdain weapons') get to look a bit like the heroes they're supposed to be at the end - thanks to an overpowered magic sword, some smiting and some buff and protective spells. Doesn't seem so bad.
Even in the end game encounter against Tiamat when my wizard couldn't do much in combat, he was the only one that was going to get us out of there if things went bad. I was sitting on my teleport just in case. I had maze ready for a short respite from Tiamat if we needed it. I believe maze is an ability check rather than a saving throw, thus allowing it to bypass Legendary Resistance. That's one thing I recommend every wizard player do: read the spell text carefully. Small differences in text can make all the difference in battle.
That doesn't sound too useless, and it does sound like having really-magical magic that can do things others simply can't.
Ironically, that you didn't need the Maze puts Tiamat in the category of 'not worth a spell.'
[SPOILERS]
What I meant was there is a way in the module to break the spell immunity and if the party doesn't they are likely to fail - but you didn't fail which is cool. And once mid level spells become an option there are more chances to try and use up the legendary resistances.
So, in theory, casters were supposed to have a shot at more fully-participating in the final battle, as well, if the party had jumped through more of the right hoops?
It sounded like they didn't just 'not fail,' but virtually rolled over the goddess of evil dragons. Maybe letting that circle protect from her breath weapons was too much?
Secondly, I'm sad to hear that WotC hasn't realized that making magic into blasting and buffing and nothing else, albeit with different skins to "add variety" was something they should have left in 4E. If there really aren't spells that work on anything other than the target of the spell, that's a pretty terrible choice that hurts not only the people who play spellcasters but really limits the game itself, because it diminishes what sorts of things can happen in adventures, not to mention discourages some folks from playing.
What you're whinging about, there, definitely isn't an issue in 5e, and wasn't even one in 4e. "Casters not as profoundly broken as in 3e" isn't the same thing as "casters useless," nor even "casters not Tier 1 anymore." Indeed, Celtavian described having a Tier 1 experience with his Wizard, right up to the until point that the module used multiple god-like attributes of a capstone legendary encounter to stack the deck against him. (And even that, apparently, could have been knocked down if the party had managed to "pixel bitch their way to the magical correct solution," as you put it.)
