Difficult certainly but catastrophic may be a tad dramatic. The plan has long been to have one QE available at any given time. Bringing the second QE into service makes that possible 8 years out of 8 rather than 5 years out of 8 but as far as I can see the "one available to deploy" ambition hasn't changed, except possibly in extreme circumstances.
Bear in mind also that the complement of a QE, excluding airgroup, is about the same as for an Invincible, despite the disparity in size. I believe decommissioning Ocean is part of keeping the manpower situation difficult rather than catastrophic, along with the 450 uplift in RN manpower announced in the SDSR.