84 year old being weaned off a non invasive ventilator with suspected COPD. What will happen next if lungs don't cope?

CPAP/BiPAP. Some patients benefit from nocturnal support with CPAP or BiPAP chronically.The patient should be evaluated for sleep apnea and arterial blood gasses should be analyzed. A persistently high pCO2 or acidosis may impair weaning. Look for complications like infection, concomitant congestive heart failure which may be contributing.
COPD. Hopefully you can maintain oxygen saturation with small amount of supplemental Oxygen with close observation.