It's not that it wouldn't be possible, but it may be impractical.

First off you 75% of the cost is power amp doesn't hold up.  You forgot labor and overhead and design. And the 75% will have to change.  You change the number of preamp tubes, you change the current draw...you have to change the power supply or add filament transformers.  You add more than a certain number of channels, you have to change out the chassis, which can mean having to deal with a vendor for a whole new chassis....design, setup, machining, prototype, refinement, etc.

As is these are all the same amp with different mods or tweaks. 

The 18-watter has three popular preamps...Traditional '60's, TMB, and EF86 (the latter two having been invented in the last decade).  Any two of these can be incorporated in the same chassis, using the same transformers.  That's how the TMB and EF86 preamps came about to begin with, the guys over at 18watt.com started playing around with the available space and were able to put in a Pelxi (TMB) and AC15 (EF86) preamp.  Now, as is, you can get any combination of two of these from Nik....Trad and trad, Trad and TMB, TMB and EF86 and there is no reason why he can't make a EF86 and Trad...all using the exact same chassis, transformers, cabinet, and many of the same parts.  But three and shoehorn all three preamps in...new chassis, new transformers, more tubes, etc. 

With the Overtone, they are all variations on the same theme.  None of these, not a one is night and day different from another...you've got your Vanilla, you've got your Vanilla Bean, you've got your French Vanilla , you've got your Creamy Vanilla, etc.  All different, all the same.  The differences are only widely apparent when compared side-by-side. 

While the indecisive, who spent too much time in the clouds dreaming up all the possible options of a setup, always seem to want the "kitchen sink" solution, the truth is eventually they end up settling on a couple of favorite options and the rest go unused.  That's why must manufacturers never get too elaborate in their offers.  So, Nik can offer four different Overtones that can all be built in the same chassis with 90% of the same parts, to meet each buyers palette.  Its just not cost effective to offer a much more complicated specialty amp that combines all of them.  An economical solution is to isolate the few components that make up the largest change in tone and make those switchable (Champ Ultra) to offer better versatility.

As a matter of fact,m I'm sure the only reason that Nik even bothers to offer so many variations of these amps is BECAUSE they reuse most of the same parts; if he ended a custom chassis for each version, chances are there would only be one Overtone available and one 18 watter. 

Aside from all of this, what is the motivation of a manufacturer to reduce the number of amps you buy?

Ultimately our hearing perception strips the details anyway.  Something sounds Marshall-esque or Vox-esque or D*mble-esque or Blackface-esque or Tweed-esque to us.  On a record you may be hearing a AC15, AC30, AC50, or Super Beatle and changes are the best you'll be able to identify it is ..."that's a Vox" because they all have that Vox-voiced preamp.

And another point:
I find that no matter how long I spend in a store with a guitar or amp,
I only really learn if I truly like it over the next month or two, when I've learned its intricacies
For amps with a more limited pallette, there is less chance I'll get the best out of the amp.
Since I recently learned that guitar stores are not the best place to get amps from (usually just the latest PCB stuff), and it's often better to mail order them from the other world, I am relying on reviews and sound clips, and again having a bit more to play with on the amp is helpful.
Having said that, I do find I'm loving the simple amps a lot, the JTM45 especially

The tone should always be in the fingers. That's why I sold all my pedals in the late 90s. I finally bought 2 again this year that I use 10% of the time, last year I got a Boss GT10 to try out - you would not know who was playing through that thing, i sold it after a few weeks.

I'm obviously guilty of spending a lot of time and effort on the equipment side at the minute, but for me it's not a permanent state of tweaking.
I've been playing with the same guitar and amps for about 10 years since my last proper search for gear. The settings on those amps didn't change much over that time, I don't change settings much once I find my sweet spots, but some of those amps didn't please me much.

I've realised that I should have looked for better gear sooner, and Over the last 2-3 years I've been revisiting my gear, with the objective of getting the best gear I can afford that suits my style, since I've realised there's no point waiting for ever. Last year was acoustic guitars, this year it's amps.
So, I've sold all my old amps except the Twin, and I've been playing with what I can find. In time, I expect to sell some of them, and you're quite right, my tone is in my fingers, and I sound just like me on all the amps, but it's fun finding which ones work best for me. With the amps and guitars, the best ones inspire me more, and sound better, but I still sound like me of course.

You must know of bikers with several bikes, or wealthy people with many cars, I know it's possible to survive with only one bike or one car, yet if one has the money, why not have many? In the case of guitar amps, I can afford to own a few, so I do collect gear, I confess.  Even if the recording can be EQ'd, the amps feel different to play.

btw I have heard of people who want a vox and a fender sound in one gig, and know one who had an amp custom-built to do it, A/B switch pedals are popular enough, so I assume a lot of people are playing live with 2 amps. I just found a plug-in preamp module from Randall on ebay today, designed so you can change your preamp in the way I mentioned in a chassis containing a power amp and a space for the preamp.

I think I'm coming at this from the opposite direction from the one you suspect, rather than someone worrying too much about amps when I should work on technique, I've been working on my style and technique for 30 years, and only now am really buying amps that are "special", since it occurred to me that it was foolish to make all the effort and then use inferior gear.
However, trying an amp in guitar shops usually means sticking with the newer models, and even then, a demo and shop-trial can be very different from the way you feel 2 months later. So my theory is "buy a few, test them for a while, then ditch the ones that don't suit", ie. run my own lengthy beauty contest.

btw you are correct, the AC15H1TV and the DZ30 sound very very similar indeed, I think that amp is Vox's re-copy of the DC30, the switches are different but the channels and controls give roughly the same results

Anyway, I think you are right to suppose demand would be too low for amps with 4 preamps. I'm able to buy different amps anyway, so I will survive.

Cheers
Tone

Fender has been doing this since the 60's. The "normal" and "vibrato" channels use separate but identicle preamps into the same power amp. I think a better idea would be to build a tube power amp and then just build/use whatever preamps you need.