You're just being obtuse. Everything that came after Judas Priest and Iron Maiden that were critical to the evolution of Metal would have, in some way, whether in a large or small role, would have been fundamentally different. You can't remove a critical component of a band's sound and claim that they sound the same as if that component was not removed, and that is essentially what you're saying when you call "CONJECTURE!!!" on my claim that bands would sound different if you eliminate key components that make up their sound. Without Judas Priest, even Iron Maiden would have come out differently, as would the entirely of the NWOBHM movement. The NWOBHM movement having taken a different path would fundamentally alter the course of the development of Thrash Metal via genre staples such as Metallica, Slayer, Megadeth, and more of the most significant players in the field. In turn, the evolution of Death Metal and Black Metal would also have been fundamentally altered. The extension goes on ad infinitum. It is called a chain of events, and is hardly reduced merely to conjecture. It's not hard to perceive that Slayer would be radically different without the influence of Judas Priest and Iron Maiden. It's fairly clearly certain that bands such as Death, Possessed, Slaughter, Sacrifice, Death Strike, Massacre, and other early Death Metal bands would have had a different approach as well due to the fundamental difference in their primary influences. The same case can be made for the origins of Black Metal. Venom would have been dramatically different without Judas Priest especially, as well as Iron Maiden, and the same can be said for Mercyful Fate, Bathory, Hellhammer, Celtic Frost, Sodom, and all of the other originators of the style. As logically follows, if you dramatically alter the fundamental core of evolution of x, then x will be fundamentally altered. It's hardly mere conjecture to follow a logical chain of events to demonstrate the way in which things would have taken a different path had a link in the chain that makes up the path which was ultimately taken been removed.
Of course, this is all assuming that, had these bands never existed, the NWOBHM movement would have still occurred, which very well could have been unlikely, and at best it would have been very different. Following this pattern, it could very well have been the case that the early Thrash Metal bands would have not adopted this formulation and would have either never existed to begin with or would have simply been Hardcore Punk bands in the vein of Discharge, GBH, The Exploited, etc.
This is also, of course, taking a complete blind eye to the obvious influence that Judas Priest and Iron Maiden have instituted on the harmonics found in, yes, Lamb Of God. So yes, you're wrong assume the probability or even remote likelihood that Lamb Of God would be the same today if Judas Priest and Iron Maiden had never existed. The likelihood is so far off the charts of probability that its actual occurrence would most likely be a result of Lamb Of God simply being an incredibly innovative band, since the path forged without Judas Priest and Iron Maiden that, in its current formulation, would have been left with a tremendous void that Lamb Of God would need to fulfill in order to make up that ground. The fact of the matter is, and this is hardly mere conjection, that in all credible likelihood, Lamb Of God would not sound the way that they do today if Judas Priest and Iron Maiden had never existed, and to assume it's likely that they would have is either to assume uncanny innovative prowess for the band or to assume a great deal in the way of plugging the gaps in the chain of events which would have to essentially artificially create the influence that Judas Priest and Iron Maiden had on everything that developed in their wake.
