It’s not a one-for-one replacement for at least two reasons: each shopper doesn’t have their own delivery vehicle, and the number of shopping trips that require vehicles is less.

Exactly. We'd be substantially better off, in terms of total quantity of driving done, if the standard practice was to have groceries delivered. Grocery stores could then be optimized around that, and eventually some of them could stop catering to in-person customers entirely, reducing expense and waste.