How could multiclass caster classes work without losing important level 6-8 spells?

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Then here’s the question:
I How could multiclass caster classes work without losing important level 6-8 spells?

— Craig Brown (@CraigBCBrown) October 9, 2017

it's tricky to balance – would need to lose some class features, because those spells are so good. limited to 1 slot each for a reason.

— Mike Mearls (@mikemearls) October 9, 2017