Other than I KNOW steel lasts, I don't see the longevity issue as a telling one. I don't buy pistols with the intent of future generations using them, though I expect they will be able to.
I've got 100+ year old firearms in perfect working order, and I'm not terribly concerned about how long a non metal frame pistol will last.
What concerns me more is what they are. And, what they are, isn't "my cuppa"...I've handled and shot several Glocks, M&Ps and similar styles and I don't care for them.
They're probably fine for people who's experience begins and ends with them, but I don't see how anyone who knows what a fine handgun is, would be happy with one.
If you're one of the "my gun is a tool and I don't give a crap about anything other than it working" guys, that's fine. I'm not.
Every metal frame pistol doesn't make my heart sing, but a few really do. Haven't seen a polymer frame one yet that stirs me beyond "meh..." and some don't even do that.
So, what's the advantage to a metal frame pistol? For me, its the fact that I LIKE THEM, and for me, that's what matters most.