Metlhed, I'll argue with you, or at least play devils advocate, if for no other reason than to learn more.
I think we can both agree that a CNMG with a 20 degree positive chip breaker sitting in the holder will give you 15 degrees as the workpiece sees it, and 70 degrees of solid carbide supporting the edge. A CCMT with a 15 positive chip breaker will show the workpiece 15 degrees in a neutral holder and 68 degrees of solid carbide supporting it. Unless your sitting in a positive holder, which I've only seen once(never looked for one).
On the unsupported edge, I just went and looked at some DNMG, WNMG, DNMG 432s(431s) some finishers some roughers, generic job shop stuff. They are ground on both sides, so that the high parts of one side of the insert support the cutting side, most are pretty close out to the cutting edge(.020-.040 or so) .
However, I also have some Taegutec(ingersol) CNMG 432 ML TT3050s, pretty darn positive, no T-land, just a medium hone, and the nearest underside support is about .115" from the cutting radius. I understand what you are saying, but I'm finding it hard to believe that the insert will actually bend signifigantly. 3/16" thick(minus double chipbreaker depth), approx .200 wide, hanging out .115". Seems far more likely that your toolholder or especially your workpiece will flex first and much further, especially on a finish pass. I have seen carbide bend and not break, so I know it can happen, but in turning the insert seems to be the most stable part of the whole mess.
A reason you might actually like negative holders, you can get single sided, very positive inserts for them when you need them, and revert back to the more common stuff when you don't.
I'm not discounting what you are saying, and have filed it away hoping it comes in handy some day, I just need more convincing.