If it works for you, who am I to rain on your parade?

But it wouldn’t be my choice.
1) I really try to avoid riding with so much gear in a backpack that an external frame backpack would be needed. Added pressure on butt, hands and arms.
2) where EF packs have their strong side is to be able to carry the load high. I much prefer keeping the weight low on the back for riding.
3) I find that the frame interferes with how I like my back to curve, and occasionally twist during riding.
4) even at 6000+ miles/year, backpack wear is quite far down on the list of commuting costs.
5) by now, I’ve gotten panniers that do the job with greater comfort and sense of control than big packs.

I commuted wearing a backpack for a year. That was enough for me, but it wouldn’t have been as big a deal if it were like 5 miles or less I think. I’d never have been on the bike long enough to sweat much and it would have meant not less strain on my body but for shorter amounts of time. For me, it was cumulative and now I have lower back pain where there was none before.

Edit: the grocery store is 2 miles from home. If I needed to grab 3 bags of stuff and had a hiking backpack to do it with - hell yes that’s how I’d do it. But I’d only use it for short occasional trips like that.