Let's break down the diplomacy feat and unpack it completely. The feat grants, after 1 minute the following two effects:
1. Advantage on diplomacy checks
2. The baddy won't attack you.
Now, let's back up a second. Any group with a rogue with expertise in Diplomacy gets the same bonus as this feat grants. A second PC with diplomacy (hardly a rare thing) can grant Advantage on every single diplomacy check. Now, since I haven't heard a single whisper of a complaint along the lines of a Diplomancer dominating the game, I'd say it's pretty obvious that granting advantage on a high diplomacy skill is not game breaking. In three years of 5e, no one has complained about this, so, AFAIC, it's a complete non-issue.
Which gets to the second part- the no attacking. Thing is, if you step back for a second, you realize pretty quickly that this won't actually apply very often. If you've used diplomacy on an NPC to change its reaction to neutral or better, why is that NPC attacking you? By the time the charm effect kicks in, the odds of combat are probably very low anyway. It's not actually going to change anything.
Where I can see this coming in most often will be in negotiations. Bartering for equipment, that sort of thing. Which is fair enough. You spend a feat, you get cheaper equipment. Cool. Smaller groups, where the odds of doubling up on Diplomacy proficiency will likely get more mileage out of this than larger groups.
But, in any case, the odds that you can stand around something for a full minute and it hasn't already attacked you means that it's probably not going to attack you at all. It's a nice ribbon, but, hardly the game breaking mess that people seem to be thinking that it is.