A 68-year-old man with severe COPD is admitted to ICU following failed NIV. He is intubated and ventilated. Initial settings: VCV, TV 450 mL (IBW 75 kg), RR 14/min, PEEP 5 cmH2O, FiO2 0.35, I:E 1:3. ABG after 2 hours: pH 7.36, PaO2 10.2 kPa, PaCO2 7.5 kPa, HCO3 32 mmol/L, BE +6. The registrar proposes increasing the RR to 20/min to normalise the PaCO2. Why is this approach potentially harmful?