Mixed mode is needed now to help to alleviate the current arrival and departure delays, without deploying additional capacity.
That sounds like the perfect description of a "lose-lose" scenario.
Delays are not the driver here - they are simply the expected and accepted consequence of scheduling at 99% of capacity for 16 hours of the day.
Nobody is going to propose a solution where the 60,000 additional slots generated per year are simply used as a buffer, without scheduling more movements.
That would certainly improve resilience and reduce delays, but would do nothing for capacity, while simultaneously upsetting large swathes of the West London population. That's not going to happen.