Okay Sacrosanct. Let's back up.
What I mean by "okay for immunity" is that the fighters still fulfil an important role: standing there taking the hurt shielding the wizard.
I don't mean it should be open season. I'm merely saying it isn't an automatic mistake. A DM that knows what she's doing can well call in an immune monster to see how the party handles it.
Since the DM is expected to be a capable one, she will not throw an immune monster against a spell-less party that the slowest member can't outrun, unless she feels confident the party will be able to think of something to outwit the monster.
I don't know: the fast fighter volunteering to draw away the monster while the slow fighter hides, or something.
I'm not arguing against your statement "you didn't need to give fighters magic items to offset a monster's resistance".
I'm merely reacting to what I percieve to be a lack of nuance to your conclusion: "You're not neutered like you were with complete immunity."
To which I'm basically saying sure, but even complete immunity (to non-magical weapons) doesn't mean complete immunity, assuming you have a spellcaster in your group.
That's all I'm saying.
The fact you might ALSO want to throw an old-school golem (immune to magic but not to pointy sticks) at the party is a completely different discussion