To be honest, I think the Cosford Air Show team deserve credit for making this decision. There are already ominous signs of a rise in infections and, whilst large events are being trialled to try and establish any correlation with infection rates, how soon that data becomes available is a moot point.
However, it's the rise in infections that is the most concerning because if they continue to rise, then at some point, despite the commercial and financial pressures, some form of lockdown will, however politically unpalatable, have to be imposed. This is hypothesis of course, however, it would be very imprudent to organise the event and then suddenly find it couldn't happen as a result, plus the loss of revenue involved in the planning and preparation.
By cancelling now, unfortunate though this maybe for the public, at least any cancellation costs can be kept to a minimum plus the public are saved from any sudden surprises.
There's always next year.