I think discriminatory is perhaps not the best word. You could argue the current lockdown discriminates on disability grounds in that tighter measures (shielding) apply to those with diabetes, heart problems, dementia, etc.
If it was for a few weeks people might accept it but it would lead to resentment and I doubt the police would want to police it. Anyway, how would over 70s shop. A compromise may just to give 'strong advice' for fit over 70s to continue to stay indoors- which is in fact what we have now