It is actually better for durability if the outer shell is removable as well. Eg, having a durable phone case on a midrange phone is better than a rugged phone. Simply being as you can easily swap the case
I’m going off of hearsay here from a video I saw years ago, but it was comparing a decently durable android in a rugged case with a purpose built rugged android phone. Both sustained similar damage but at least the one with the case could be fixed by a case swap. Other one would need a full on repair.