Every year Apple keeps making thier phones thinner. But then everyone gets cases that make them thicker. Wouldn't it be better if they made the phone thicker and stronger and less slippery. This could also give more room for a bigger battery.
Not every year. They only get thinner as they get bigger screens. Thicker would make a Plus harder to hold with one hand and heavier. The battery life on it is incredible. I don't think I've ever heard any complaints about it. Of course, even longer battery life would always be nice, but not at the cost of making a large phone heavier or more difficult to hold. Also, the trend with other manufacturers is thinness and Apple needs to stay competitive.
The people who use cases now would probably still use cases even if it was 50% stronger. Unless it's indestructible, many would still use a case. You can't make it indestructiable without making it less asthetically appealing, and the sleek look is extraordinarily important to Apple. It's unlikely they will ever rank that behind durability. We would need economical shatterproof glass to exist before we would see a change here, in my opinion.
Now, I did agree with making the non-Plus 7 thicker so it could also have decent battery life. It's small enough that it would still be easy to hold with one hand and weight wouldn't have been a problem.
The Jet Black is much less slippery. It's the smart choice if you are going to go caseless.
Some of this is a moot point because the next phone will likely have an OLED display, which will improve battery life without increasing the size.