Mobility and motor capacity are incredibly easy to lose - many reasonably fit folk in their 70's and 80's will emerge from 3 months' isolation unable to walk more than a few yards. Quite a few will never recover their previous levels of fitness - that's got big implications for them, their families and the care system.
Not everyone has someone who can shop for them. Not everyone has the technology to shop online. Lots of folk have poor credit records and can't just flash the plastic as supermarkets want us all to do. Many are, or will soon be, running out of money and unable to afford the stocking up needed to survive months of isolation. More than a few, I suspect, will die alone at home and not be found quickly.
Like you, I've set up my own network to check on folk I know who live alone but, yeah, a lot of people are going to fall through the cracks.