Exactly this. As an absolutely desperate measure, asking people to wear masks were it may not be possible to distance might be OK for a short term plan. However where this all starts to fall down is why masks are even part of the equation. It has long been forgotten that in some Eastern Asian countries, it is a social protocol to wear masks when displaying symptoms of an illness, such as sneezing or coughing. As with the policy here this is not to protect the wearer, but those around them. However with covid people are being asked to wear them regardless of symptoms, because there may be a short window prior to any symptoms where it may be possible to transmit. However if the person not showing signs of illness isn't coughing or sneezing, spitting or licking the windows, then the risk of transmission remains incredibly small, as it would with other illnesses that transmit in similar ways.
So this then begs the question, when can we stop wearing them? Let's say it becomes law to wear them on public transport, what will be the trigger to stop wearing them, because after all if we are wearing them to prevent a very small chance of transmission if not displaying symptoms, and given that the virus is never going to disappear, then its hard to see what would allow a relaxation of such a law. Indeed it is probably one of the reasons why the government hasn't gone down this line, because there are serious implications.
As
@NorthOxonian rightly points out a situation like this could lead to long term stigmatisation of public transport, and because culturally we are not used to seeing people with masks when other relaxations kick in but public transport users continue to have to wear masks, it will potentially discourage people from ever using it again. This will lead to a shrinking of the networks, and invariably loss of jobs. Perhaps in time we will move to a cultural shift as in Eastern Asia, where people wear them when they are suffering, or better still we become more tolerant of people staying at home & being able to have confidence that we won't get disciplined or sacked. But I think in the short term any compulsion of mask wearing / face covering must be very limited, be under constant review
and have a clearly defined end game.