I did a bit more research and found this is far, far, FAR from the whole story. For example, in 3.5e, the dominance of spellcasters had a lot to do with their ability to buff themselves sky-high, which is gone from 5e thanks to the concentration mechanic and reworking of buffs, killing CoDzilla. Also, spells no longer gain power automatically (outside cantrips) and non-spellcasters received significant buffs that make them more dangerous and survivable, such as barbarian rate halving damage received and rogues getting unlimited 3e style defensive rolls with no save needed at level five. That last one will be interesting to see implemented, actually.
Charm and illusion spells will still mess up your fighters on higher levels. Magic was never really about dealing damage, it was about control, and casters still have that covered.
Except I've been doing some reading on this and it's a lot more than just control. Control was part of it (and now is the largest part of it) but it was far from the only aspect. Spellcasters had far more and better damage options for AoE than fighters, plus being able to buff themselves into being nigh-on physical gods. I'm going to go ahead and quote
1d4chan here, on the topic of linear warriors, quadratic wizards.:
Dungeons & Dragons 3rd Edition (and Pathfinder) - The first case of this being shown. All casters benefited from their casting stats contributing to how many times they could cast in a day, all spells scaled with the caster's level, and spells were so potent that you could actually cast them during a turn without risk of losing it while prepping it. Also present is metamagic, which allows a caster to add extra effects to a spell for the cost of a higher spellslot than intended.
...
Dungeons & Dragons 5th Edition - Also attempted to solve this problem with less overtly new mechanics. Their solution was, firstly, to remove the idea of spells automatically increasing in potency due to their caster's level except for cantrips; in this version, you have to sacrifice a spell-casting slot of higher level to increase the might of your spells. They also removed the "chain of buffs" approach that had been integral to CoDzilla; most buffs became "Concentration" Spells, preventing you from casting more than one and also making it riskier to go into battle whilst buffed yourself. Finally, "minion mastery" spells were heavily nerfed.
Meanwhile, martials are fundamentally similar to their 3e counterparts but with major defensive buffs: fighters can replenish a chunk of HP in one turn and reroll saving throws, barbarians roll twice for dex saves and rage now halves damage from attacks, and rogues get the benefits of 3e Defensive Roll at level 5 but with unlimited uses and no save needed. Casters still have the edge in sheer versatility, but over time high-level martials can match their damage output and are much harder to kill.
Also, there's an interesting discussion on
Reddit about how much spellcasters (specifically wizards) changed between 3.5e and 5e. Basically, in the former, they were essentially gods over all others (aside from clerics, druids, etc.). In the latter, they're much closer to the other classes in terms of power. This comment from peacefulelm in the reddit discussion sums it up, I think:
Wizards in 5e are designed to augment the other party members when they "do their thing". Wizards help rogues be better rogues, fighters be better fighters, etc. They can also, I'm a pinch, take on someone else's role (but without any of the style or finesse that specific class brings to the role). Wizards can open locks, but not silently. Wizards can cast sleep on a crowd, but it is a waste of a spell if they don't let the fighter or the rogue whittle down some hp on the baddies first.
In 3.5/Pathfinder, they tried to follow the same path but it got out of hand. Wizards ended up being better than everyone at anything they put their minds to. Greater invisibility and zones of silence negates the rogue (and doesn't really cost the caster that much). A single summon spell left the fighter twiddling his thumbs as beings that could die without cost went into the fray instead. Wizards stopped helping out and started just solving all the problems by themselves, leaving every noncasting class in the dust around level 6 or so. Most of the game was waiting for the wizards and other casters to win the game for them or overcompensating by building super OP charge builds to stay relevant (while the casters still were able to do everything without a problem or much work). An unoptimized caster beat an optimized martial player or skill monkey at least half the time.
To someone who liked 3.5/Pathfinder, 5e will seem like a nerf. Casters have less spells and can't have as many going at the same time. And technically, it is a nerf, 5e casters aren't the gods of everything that they were in the past. And that's a good thing. Your friend just needs to understand what the new wizard is, and the new wizard is everyone's best friend instead of a genie out of his lamp. He'll have to get used to it but he'll come around.
Edit: if this goes on too much longer, it might make sense to split it into its own topic.
