I think it is fair to say that Slayer is still having a profound effect on metal.
On this your guess is better than mine, literally. You are well-versed and follow various aspects of the Heavy Metal 'scene' more than I have or will, and when I say they're irrelevant I am talking irrelevant largely to the sounds that reach my ears. I don't think any of the bands I am interested in have been affected by Slayer after Undisputed Attitude or thereabouts, and those that have been certainly not in a positive way... in that some bands might have gone 'well if Slayer becomes softer to make it and still retains their 'cred then maybe I should as well!' for all I know. It may not be scemantically sound, but I use 'relevant' as a positively charged term.
Where we end up talking about two different things is that I am not looking at the whole of the Heavy Metal scene, my eye is simply not wide enough nor do I have the time or inclination to really know to what extent the new record by a useless to me Slayer is relevant to useless to me bands. I appreciate that a holistic point of view, that includes the crap in the equation is probably more representative and useful for historians and people who want to have a clear picture of what happened, when, and why. I appreciate that you do this. I certainly reap the rewards of such a thankless task by reading and learning from people such as you far more about the industry of metal than I ever could have known otherwise. But I certainly keep an ear on good metal much more than I do on the industry on the whole. I don't read blabbermouth, I don't listen to new stuff unless reliable sources tell me it's worthwhile, or it's something I know I'd be interested in (I have specific metal kinks, like technothrash) and so on. I function in the 'scene with another metal credo: the bad or mediocre music can all go to hell.
There's just no time for me to analyse a bad track by Slayer, its social ramifiations and the scene status that surrounds it. I'd much rather do it for a good track by a good band.
The argument that metal develops in a rigid linear fashion is a bit strange to me. Some of the most complex and convoluted ideas about metal are very simplistic since they treat it as if it operates like a train traveling on rails. First stop: NWOBHM, Second stop: thrash, Third stop: death metal etc. etc. etc.
I understand you might be talking a bit wider here than with me directly, but I haven't thought there was a clear progression of metal for a long time. I am well aware of at least three different eras where 'power metal' ment something almost completely different, sometimes at the same time, and I've been 'dissed' by younger listeners when I've said Nevermore were a power metal band, or that Mercyful Fate were a black metal band, so on.
The clear 'Heavy Metal genre lexicon' I think solidified out of the rise to prominence of popular metal press. It's helpful to 'sell' people similar-sounding bands, and as a self-fulfilling prophecy, once very clear genres were there, people started making bands that sounded like 'generic genre example'. I think this is because if you try to sound like Mercyful Fate, chances are you'll end up sounding quite different to them in the end of the day. A lot of old Heavy Metal bands had the same influences but didn't sound the same at all. But if you try to sound like a 'thrash metal band' you can take equalized, dumbed down, basic elements of the sound and indeed arrive to sound like a generic thrash band.
I started listening to metal while reading Metal Hammer GR back in the day and when Hammer said Hallow's Eve is a thrash band, I knew that Hallow's Eve is a thrash band. It took a lot of years and personal interest in metal of old to realize that things were not once as clear as they appear now, and Hallow's Eve have very little to do with Anthrax after all.
Any way you look at it though, just through sheer number of interview references to Reign in Blood, that album has been extremely influential inside metal and outside of it (hardcore punk). This doesn't support any railroad genre argument, it's just a general observation.
Slayer is a very "big pig" as you put it, and metal is an ecosystem of sorts, so to dismiss the effects a band has on a genre because they are popular or prominent seems a wrongheaded to me.
I am dismissing later Slayer because it is BAD, you know? It's a metalhead instinct. From my point of view, I don't concern myself with bad music, why would I? Simply no time. I appreciate those that do it for some point or another, to fuel an end I might enjoy. But I won't listen to bad Slayer. I don't particularily enjoy Reign in Blood either, but it's a bearable record for me. I prefer South of Heaven. Later stuff simply started to become less and less interesting.