I'm going to second the opinion of many other posters here and thank GMforPowergamers for taking the time and effort to organize and run this interesting experiment. That said, my take on the results is slightly different from most of the opinions here. The way I see it, this TPK proves that, as in every edition of D&D to date, how prepared you are can make a key difference between an easy encounter and an impossible one.
Take the Wizard, for instance. A lot of people have pointed out that the Wizard's spell selection was less than optimal. While I agree that, in principle, spells you absolutely must have shouldn't exist, it cannot be denied that, for instance, giving the Wizard Shapechange could have made an enormous difference. My reasoning for this claim is that, assuming the Wizard had seen one during his lifetime, not a huge stretch since he managed to reach level 18, he could have used it to turn into a Couatl. This would be incredibly useful in your scenario for the following reasons:
1) Couatl's are immune to bludgeoning, piercing, and slashing damage from nonmagical weapons. Those 300+ commoners in the town? They would have been completely useless.
2) Couatl's have a fly speed of 90ft. All the magic weapons you gave the townsfolk were melee weapons. Flying allows you to stay out of reach of them.
These two factors combined would have made the Shapeshifting Wizard essentially invulnerable in this scenario. The Shapechange spell lets you cast spells in your changed form. The Wizard could have stayed 100 or so ft in the air, casting Magic Missiles into the village below with impunity. The only threat would have come from the Assassin's crossbow, which can deal Poison damage, but even then, a Couatl has a base AC of 19, which can be boosted by Shield to 24, and if the Wizard flew constantly outside of the short range of the crossbow, the attack roll would have to be made with disadvantage (or normally, with surprise) and the Assassin would probably only have one chance to break the Wizard's concentration before being neutralized by a spell like Disintegrate. Given that Shapechange can last up to an hour, 600 rounds in other words, the Wizard really could have, with a more than reasonable chance of success, soloed the whole scenario with the help of that spell.
Then again, the tactics above were an ideal solution I came up with knowing the ins and outs of the entire scenario. There's no guarantee that given Shapechange, the Wizard's player would have reached the same conclusion. Still, even if he used it to turn into an Adult Red Dragon or something similarly deadly and, more to the point, with the ability to fly and with high AC, it would have, at the very least, been more useful than Time Stop.
As for the lFqW argument, my experience with 5th Edition D&D so far is that when it comes to comparing classes with each other, the grass is always greener on the other side. Sure, a Fighter's player might envy a Wizard's immense variety of flexible options, but at the same time the Wizard's player might envy the Fighter's ability to deal more single target damage at will than the Wizard can by expending spell slots short of Meteor Swarm, as well as the fact that the Fighter doesn't have to worry about Magic Resistance or Legendary Resistance.