So do you think that if everyone using the Underground wore a mask then the commuters would return in large numbers?
I assume no such thing - much as you and the Mayor would like to believe otherwise, I very much doubt that mask policy makes anything more than a marginal difference to Tube loadings - of those I know, I can think of one person who might be influenced by mask policy as to whether they travel on public transport or not.
The landlord at my employer's office in central London is limiting lifts to a capacity of 2 (from memory, the nominal capacity is 12). When I was last in, there were no queues because of that. Pre-Covid, with those 5 lifts full, you could still queue for 5 minutes at busy times. It's lack of commuters which is holding passenger volumes down on the Underground, not whether or not masks are a condition of travel.