Concentration for Martials

[Amnestic]

I kind of think/hope this was said in jest. Abilities that can't work if characters have another ability (not are using that other ability, but have it), are a bad design pattern.

Not sure I agree with that honestly. The extra attack issue has always stuck out primarily because it's entirely a balance ruling. There's no in-universe reason why a Fighter 5/Paladin 5 has only 2 attacks instead of 3. A dead level solely for game balance feels like the designers didn't plan properly.

But there might well be an in-universe reason why Spellcasting and Martial Focus (or whatever you call it) are incompatible, and that's a lot easier to understand. We already accept (sometimes begrudgingly) that while a barbarian is raging they can't cast or concentrate on spells. Same sorta thing.