The spike protein is under selective pressure. It needs to bind to ACE2 in order to reproduce. This constrains its mutational landscape. The viral envelope, for

The spike protein is under selective pressure. It needs to bind to ACE2 in order to reproduce. This constrains its mutational landscape. The viral envelope, for example, isn't similarly constrained and can undergo neutral drift. It would probably be wasted effort to target these with subunit or mRNA vaccines. This isn't an argument for challenging vaccinated people with Delta, but suggests there's room for a live attenuated or whole inactivated vaccine booster.