I agree 100% with what you are saying, but Sainsbury's are also contacting the Government list of vulnerable people. Just add in Tesco and Morrisons to do the same and the market is cornered. Theoretically a person on the list could shop at all four stores separately denying three others the opportunity to shop at all.
The list should have been co-oridinated and people given the opportunity to select which store they are going to use freeing up slots at other stores.
The more people we get using online delivery means there are fewer people able to catch the virus, which helps everyone, vulnerable or not, in the long run.