Well, I mean, yeah. That's the unspoken main reason, of course.

Watermelons, messenger bags, and hilly areas are the worst mix. I think you get the idea.

my mission workshop backpack is actually better than my ortliebs and a rack. it's more stable fully loaded and holds more.

I use PAC Ultimate (one size down from the O.S. Ultimate) to buy groceries just two days ago. The size of the bag expands along with expansion straps that allow me to close the bag and ride with no issue.

Groceries, a large backpack style with roll top or large sized messenger bag by most makers would work. The Pac bag I have also has a second X-strap to distribute weight over two shoulders but I seldom need to use it.

If you are going to go this route and have any big hills on your way home then I would recommend two brakes. Bombing down hills with 50 pounds of groceries can be scary as ****. For big hauls I prefer my geared bike. Currently my grocery getter is my own two feet though.