This falls into what I said before about having to tailor your encounters to include things for the lower levels to do. If you don't take the extra step to do this, the low levels are in peril.
Well, I'm not tailoring the encounters. The encounters are the same whatever the pcs are. If they go to the dungeon that's run by bullywugs, there are the same 2 bullywugs guarding the entrance whether the pcs are 1st or 20th level. Likewise, if they go to the Burning Mountain, the same four CR 13 robots are guarding the entrance, no matter who or what level the pcs are. The pcs use their knowledge of the local area, rumors they have heard and obvious clues ("nobody lives within 20 miles of Bile Mountain" vs. "the abandoned monastery is just out of town; whatever is in there can't be too nasty or people would know/be dying") to choose their own adventures.
I really do run a classic style sandbox in which I focus on setting instead of plot.
Also, fireball is honestly just an example, there's plenty of worse spells, and low level characters actually have worse saves, meaning they'll be affected by the bad stuff more often, potentially taking them out of the fight.
Also, I notice your examples for how low levels contribute were mostly armoured, tough defender types. What about the squisher characters like rogues, sorcerers, warlocks, etc? They could be forced to stay back and plink away at range, which would certainly suck for the rogue if his player wanted to make a dualist or swashbuckler or something.
Well, I will use last night's game as an example. I'll focus on the two lowest-level pcs. The party was:
Dzedz (pronounced roughly "Zed" unless the next word starts with a vowel, in which you hear the final "z"), 7th level evoker wizard
Seraphine, 5th level arcane trickster
Morsado, 7th level bard of lore
Bruford, 4th level battlemaster fighter
K-Wild, 1st level rogue
Lygress, 3rd level warlock (with Harriet, a pseudodragon familiar)
So a party of 1st to 7th level pcs. They started off just outside a dungeon they had delved last game. It was after dark and they were pretty wounded. They set up camp and had one random encounter with a giant poisonous snake during the night; a quickie, if you will. This was before the 1st level pc had met up with them and was a single-hit kill, but did some damage to the arcane trickster.
During the next day's journey, and with their new 1st level pal, they were attacked by a fairly massive wolfpack during a period of extremely dense fog (20' visibility). During this fight, due to the terrain and the limited visibility, the wolves didn't all get close enough to attack until round 3 (though most could attack by round 2 or 3). The rogue did his part, sneak attacking effectively. The warlock used her
eldritch blast to good effect. Both contributed, and I think that the warlock took a little damage (like 1 hit's worth IIRC).
After that, they encountered a needletooth drake swarm- a custom CR 6 monster converted from the 3e MM3 IIRC. This was a nasty encounter that nearly took Seraphine out. The warlock did her part, but in this one, the rogue did a lot of quaking in fear because the character had seen a swarm of them eat a bunch of children from his tribe in the past, so he was terrified and roleplayed it. So in this case, the pc didn't contribute much tactically, but did do some great roleplaying.
Their next encounter was a chase/pursuit/tailing encounter. They didn't know it at the time, but their quarry was a doppelganger. They caught him changing form with a
crystal ball and chased him halfway through a city before he managed to give them the slip. This was followed by some investigation and the slowly-dawning realization that Dzedz' mom was a werewolf.
Then they were ambushed by a cadre of
8 doppelgangers in the street. This was a vicious battle, although the 1st level guy was now 2nd. The party came really close to losing this one; by the end, Seraphine and Bruford were both dying (Bruford had 2 failed death saves), and only a combination of good tactics and really poor damage rolls on my part on several hits that really ought to have dropped one or another of them kept them from going down as a group. In this fight, the lvl 2 guy (now a rogue/monk) did a good job of making sure he landed his sneak attack almost every round (using a dagger with his bonus action attack as a monk). The warlock made good use of her
eldritch blast again, though she had a rough couple of rounds when two of the doppelgangers mashed her from either side. (Wise decision time: she Disengaged.) Though neither one dealt the same amount of damage as the wizard or fighter, they both definitely contributed, and each one dropped at least one of the doppelgangers (well, the pseudodragon familiar of the warlock got one; I don't recall whether she got one herself, but I would count a familiar's kill for her). One major element of this fight was that, once the evoker dropped a big bomb, about half the doppelgangers moved to focus on him, almost killing him but allowing the other pcs to focus their own fire a little bit better.
The last combat encounter of the night was another big burly one, with the pcs getting attacked by two giant apes while in the forest (looking for Momma Werewolf). Again, both low-level pcs contributed; the warlock got smashed down by one of the giant apes and dropped, but lived through it. The rogue kept lucking out and getting missed (and, of course, not every one of the apes' attacks was aimed at these two by any means; the fighter moved out front to engage them first, and held against them for several rounds before dropping).
Maybe it is partly our gaming styles. I use casting type enemies in nearly every encounter. I find that's the best way to balance the fun and usefulness of every character and class. It makes sure that AC doesn't become the only thing a character worries about, but their saves as well. So I guess if you almost never use mages, or when you do, you make sure you never aim at the rogue annoying your mage by shooting him in the back, you wouldn't see my concern.
I only use mages/casters when appropriate. In this case, the pcs were engaged with enemies who were not heavy on the spellcasters. Had things gone a little differently during/after the chase, they certainly could have ended up in battle with spellcasters, however.
But I don't have many wizards as wandering monsters in the forest or in largely uninhabited hills/mountains. I usually have a good clue where such npc types are to be found.
That said, when a party encounters a hostile spellcaster, I absolutely don't pull punches (unless the npc would do so for some reason). But you are probably right that using spellcasting foes relatively sparingly probably has an effect on survivability in a mixed-level group.
