Half-casters, and to some extent even third-casters, can fill in for a full caster. It might be rough if there's some expectation of being able to cast Raise Dead at character level 9-10, being able to cast Teleport or Plane Shift ever, or even having access to Revivify or spells like Conjure Elemental. Generally, it's only rough if the module/world expects you to be able to use those sorts of magical effects at certain levels; good scenario writers won't make that mistake. A full group of half-or-less casters can still fight, use AOEs, heal up, sneak and sleuth, and (with a feat) cast useful rituals.

Full martials (a spell-free team, other than racials and rituals) is much harder to deal with, for the reasons touched upon earlier in the thread. Be glad you're not dealing with that scenario.