He mentioned three possible reasons: Mechanical support (which I agree with him is plain silly), a way to increase the "yield" by using otherwise unused dies, or his conclusion that Threadripper is more of a cut down Epyc than AMD admits that electronically may allow for four full dies even if the platform remains locked to 4 memory channels and 64 PCIe lanes. What other "dozen reasons" are you thinking of that he could still be wrong?
Honestly, I think it has to do with the way they build and test the chips. Before I get into this, let me say 2 things: I know only a bit about how CPUs are built these days, but I'm also an engineer and I'm pretty good at making educated guesses. That being said, feel free to take this with a grain of salt.
The cheapest way to build anything is to keep as simple and as straight forward as possible. This means keeping all of the components the same, making very few variances, etc. This would mean that every Threadripper CPU, whether it be the 1900x, 1920x, or 1950x would be exactly the same, and they are. So why did they use 4 dies instead of 2? To increase the overall yields of the chip (I believe this was one of the things he mentioned in this video). It gives them the most flexibility. You only need 2 dies, so if one or another is defective, you can turn them off. If you want to build a 1950x, you just need 2 fully functional dies. If you want a 1920x, you just need 6 cores on 2 dies to be working. If you want to build a 1900x, you only need 4 cores on 2 dies working. If they do things this way, that means that yes, you'll waste a bit more money, but your overall yields for the chip itself are almost guaranteed to be 99%, so it pays off. These chips aren't costing AMD a bundle of money to make. I guarantee you that even the 1950X costs AMD less than $200 to build. So wasting a few dies in each chip isn't a big deal.
Now here is the kicker: I believe that his point about a 32 core Threadripper CPU is also valid. However, they decided not to do so for Gen1 due to TDP and lack of a use case and limited boost technology. A 32 core threadripper CPU would almost certainly need much lower clocks in order to make TDP. As it is, without overclocking, a Gen1 Threadripper quickly drops down to 3400 MHz when placed under load in order to keep within TDP. If there were twice the cores, you would almost certainly need to set the base clock much lower, maybe even by half (1.7 GHz for 32 core) to stay within TDP. Now, for Zen+ on 12nm, the situation may be a bit different. They've reworked precision boost, and we will also see slightly better voltage/clockspeed curves. This would likely help raise the base clocks without raising voltages. If they can optimize things enough, we could potentially see a 24 core Threadripper or maybe even 32 core Threadripper. AMD actually has competitive reasons to release at least a 24 core Threadripper to one up the 7980xe.
Now, let's talk about the 7980xe for a moment. Excluding overclocking, here are the stats:
Cores: 18
Threads: 36
Base Frequency: 2.6 GHz
Max Single Core Turbo Frequency: 4.4 GHz
Max All-Core Turbo Frequency: 3.4 GHz
Now lets compare this first to my personal 1950X:
Cores: 16
Threads: 36
Base Frequency: 3.4 GHz
Boost Frequency: 4 GHz
Max stable all core OC frequency: 4.2 GHz (4.4 if I'm willing to kill my chip from 1.6V)
Keep the above in mind for this next part. Zen+ should bring around a 5-10% improvement in clock speed. In addition, it features a reworked precision boost that should operate a lot more like Intel's chips. Knowing this, I imagine we'll see something like this:
Zen+ 8 core (2900X) (and also the AM4 2800X)
Base Frequency: 4.0 GHz
Max Boost Frequency: 4.4 GHz (all core)
Zen+ 12 Core (2920X)
Base Frequency: 3.8 GHz
Max Boost Frequency: 4.4 GHz (all core unknown)
Zen+ 16 core (2940X?)
Base Frequency: 3.6 GHz
Max Boost Frequency: 4.4 GHz (not all core)
Zen+ 24 Core (2960X?)
Base Frequency: 2.6 GHz
Boost Frequency: 4.4 GHz (not all core)
Zen+ 32 Core (2980X?)
Base: 2.2 GHz
Boost: 4.4 GHz (not all core)
Now, you may be doubting the above, but it's really all about math, what we know about current leaks, and knowledge of current hardware. Ignoring TDP, Ryzen is already capable of hitting 4.4 GHz. It just requires obscene voltage to do it due to the LPP process (I can show you a screenshot of my 1950x at 4.4 ghz if you'd like). 12nm is supposed to be performance optimized, so the curves should be much better. That being said, we will probably still not see very high overclocks. The other improvements to Zen+ should mean a slightly higher IPC and much better gaming performance though. Overall, I expect that this next Ryzen release is going to bring MUCH more to the table, more than anyone has ever imagined.
Remember this post as we go through the year. AMD may be a bit more conservative about clocks, but I imagine these numbers won't be far off the mark. We'll see at least a boost clock of 4.2 GHz and I'm putting my money on 4.3-4.4 GHz as stated above. I'm usually pretty good at figuring these things out. I was mostly on the mark about the 1950X and people said I was full of shit. I claimed that the final chip would be around 3.6 GHz base with a 4.0 boost. The end result ended up being 3.4 base and a 4.0 (xfr 4.1) boost, but that I suspect was to make the chip easier to cool more than anything.