This has been promised for a long time, but from my perspective, I don't really see this getting any closer to reality.
But for what it's worth, I think there's a conflict of interest here. No one is going to believe that Amazon won't prioritize Fresh or Whole Foods for these orders - even if they don't.
I use Ocado for my groceries. By default they will deliver based on a combination of things I've marked as "every X unit of time" and a predicted/suggested set based on past purchases. Then I can log in before a deadline to amend it. 90% of the basket typically remains unchanged, and quite regularly I can't be bothered to even log in to check what it's selected for me because I know it'll be close enough unless there's something specific I absolutely need this week.
No reason why Amazon can't do something similar. They only need it to be "good enough" that people start getting used to not always needing to ask for it to list what is in the upcoming order.
How is this any different from a physical grocery store deciding on what to position on the ends of the rows due to incentive? People love to act like these problems brought on by the internet are new but all to often it's just an evolution.
At least my grocery still is reasonably organized and well laid out. If I need ketchup, I know exactly where to go to find all of my options side by side.
If I tell Alexa to order ketchup, who knows what brand I'll get or what size? It's possible I might end up with banana ketchup or even mayonnaise given how poorly Amazon's search functions work.
The only kind of people I imagine would be okay with a 3rd party blind shopping for them would be those who haven't seen the inside of a store in years because they have help that does all of their shopping and domestic work already.
it may also have to do with price insensitive customers.... if you doordash every meal, even groceries from the highest bidder remain cheap in comparison.
Eventually people will say "Alexa buy my groceries"
Alexa will buy from the highest bidder