I'm A Banana
Potassium-Rich
The second point is the reason the fighter analogy doesn't really work. The fighter does hit more often as levels get higher...but the consequences of those hits change very little. In fact, damage as compared to a percentage of the characters health goes down. The fighter has to hit with more attacks in order to do the same amount of damage.
Which is why the fighter gets more attacks, right?
The problem at high levels is its a double whammy....the spellcaster hits you more easily AND there effects are even stronger. If the spell effects didn't increase in severity there wouldn't really be a problem. But because they do, that is the concern.
What spell effect is so severe that it is not permissible for a wizard at the height of their power to almost always succeed at casting it against a target who is literally as weak to it as the game will allow?
Should Meteor Swarm miss that guy with an 8 DEX? Does it deal too much damage for them to not hit that guy pretty much every time? Is Imprisonment something you want that 8 WIS doofus to be able to resist more often than pure luck will allow? What happens if you let those spells take effect against the most susceptible of characters? What problem does it actually cause in play if those hit?
That said, we are concerned about the poor fighter but the effect doesn't end there. For example, many high level wizards will take a power word Stun at full hp and still be affected with no save.
If that wizard has an 8 CON and doesn't make friends with anyone who can buff that, then that wizard is asking to be stunned for an encounter. Much like the 8 WIS Fighter is asking to be imprisoned.
I think it's OK to have weaknesses and be susceptible to things. It's fine to have to pay attention to areas where you're likely to be hosed. It's acceptable to ask players who are afraid of having their weaknesses exploited to put resources into shoring up those weaknesses when they go up against high-level threats if they do not want to get slapped around by the Lich (or whatever). I don't think that all characters need a strict 40-60% chance to resist every effect in the game. It's fine to have a little binary swing in the high levels.
I mean, the party with that 8 WIS fighter also has a 9th-level dispel magic that can end that imprisonment in a round. The party with that 8 CON wizard also has a Holy Aura. the party with that 8 DEX meteor-bait also has a Mass Heal and a True Resurrection. When you're theorycrafting in a a void where one-on-one combats are all you're looking at, it's easy to forget that falling prey to a spell isn't the worst thing in the world, and, in fact, it's fairly easily fixable.