I'd also like to see research on it, but ultimately, pretty much every large company went in that direction for a reason. The phone market is pretty competitive, if users preferred a replaceable battery to a thinner phone we'd see some company stealing market share by offering exactly that.

A thin phone with a phone cover is thinner than a bulky phone with a phone cover. Maybe the fact you're going to be putting a cover on it is why you want it to be thin in the first place.