I think it's relevant to consider something like air pollution. Depending on which figures you choose to accept, air pollution in London is responsible for 4,000 to 9,000 premature deaths per year. These numbers are in the same order as it appears will die prematurely from Covid-19.
Anyone who does anything during normal life that contributes to air pollution is "costing lives". Anyone who drives to B&Q, or has a barbecue, or uses a bus to go to the cinema, or gets an amazon delivery, or any other number of "wants rather than needs".
Some attempt could probably be made to quantify the effect of driving X miles, in terms of its contribution to the overall air pollution emergency. We can't as far as I can see quantify the increase in risk associated with going outside for exercise whilst being careful to maintain social distancing and so on. It seems entirely plausible to me that it is a lower risk than that associated with (say) going out in a car in "normal" life.
So I think I'm going to continue to resist those who want to try and guilt trip people about going to for a walk in the park for an hour, or going for a 2 hour bike ride once or twice a week.
And when this is over, I'll continue to believe that we should get rid of private cars in London, but not buses, because you can never reduce risk to zero whether it's normal life or a pandemic.