I read posts like this all the time on BF and I just have to wonder how much of this is based on reguritated hearsay.
I have a Ranger RD7 (5160 @ 58 HRC) that I thinned to 10 degrees per side and have extensively chopped and batoned it through the nastiest, knotiest, hardest hardwood I could find with zero edge damage other than minor loss of sharpness (it still shaved hair). It sharpens up beautifully, whittles hair, and with minimal effort. The difference between 10 degrees per side and the 20+ degree per side edge it came with is night and day in terms of performance.
I also have a chef's knife in D2 that I and my wife use almost every day in the kitchen. It has a 10 degree per side edge bevel on it and has never chipped and holds it's edge extremely well. It also takes a nice polished edge that will whittle hair.
No doubt a thinner edge will cut better, you simply have to cope with the fact that it IS more susceptible to damage, regardless of the steel. I certainly don't advocate 40 degree edges outside of cold chisels, but have seen enormous differences between 18-20 degree (included) edges and 25-30 degree edges in most steels, especially under impact. I've found this in knives, machetes and axes all the same. Batoning is not as hard on an edge as chopping, because it's a much more controlled cutting action. Chopping, you're trying to hit the right spot over and over again but no matter how good you are you'll have some amount of lateral force from glancing blows that you just won't see in batoning unless you're too drunk to be trusted with a knife.
That's why you can drive a very fine edged chisel through wood with a mallet without hurting it, but try doing some icepick stabs with it and see what happens to that edge.
As to the kitchen knife, that geometry is absolutely correct for that type of use, and yes even with D2. I've never understood people having a problem getting D2 blisteringly sharp either, but there are other steels that will hold it better at that geometry.
M4 is a really nice steel--I have a Spyderco Mule in it and also a very nice hunter/skinner by Phillip Patton that I had fun using on four antelope in Wyoming this past October (no, I didn't shoot them all, just got stuck with the dressing 'cause I'm the fastest, which is a sad commentary on our group) and it performed extremely well. I think he landed on 63.5 HRC if I'm remembering right. Anyway, despite having a very fine geometry and it being very cold, there was no significant dulling or damage even from sawing/slicing through the neck down to the spines. Never sharpened it during the trip, just made sure to get the blood off of it as fast as I could, and even then it had developed a light dusting of rust by the time I finished the first animal, but no pitting. I was hoping to have a "run off" against one of the other guys using my Dozier skinner but honestly there wasn't enough cutting to be done to phase either of them. Included edge angle is floating right around the 20-22 range, according to my little angle finder and magnifying glass.
So no, steel doesn't become glass if you drop down into thinner edges, but there's no way to get around the fact that durability is lessened more the farther you go, and different alloys have different peaks/valleys of behavior. My little D2 Queen Canoe (back before I'd heard about D2's behavior) was a disappointment to me at first, because it hardly seemed to hold an edge as well as any of my old Case CV blades; but, I'd gone too thin. Where D2 wasn't keeping up with essentially 50100-B with a very thin edge (I don't know the angle I was at it's too long ago) it easily surpasses it when both are in the "medium" geometries. Everything's a tradeoff, unfortunately.
INFI's good, but it can't do anything much better than properly heat treated 5160.
MUCH better, no, but a little better in pretty much every way, yes. The truly remarkable thing about it is that it acts like 5160/L6 and it's damned near stainless--even after seven or eight years with the stuff, that still just almost doesn't compute for me.
I've heard it a lot of times, nobody is gonna make special steel for a maker, they're too small etc...
