He doesn't have to wait for A350s. There is B777-300ER availability
even sooner. The issue is not simply aircraft availability, it's aircraft availability at a distressed price (like the Ryanair 737 order after Sept 11 2001). From Ryanair's point of view, demand for long-haul widebodies (and pricing) is distressingly robust at present.
Hardly a surprise as no way the Gulf carriers were going to pick up on all the orders they had out there.