I think with the Studio series, your average guitarist is going to walk into a guitar shop and try them all. They'll quickly realise whether they want an unapologetic Plexi, a smooth JTM45, or something with a master volume like a JCM800 or Jubilee. I think if they're considering a SV20, they're probably not expecting it to be a quiet and flexible amp, and if they want something more tamed and versatile, they'll just get one of the other models. For the minority of people that ABSOLUTELY MUST have a master volume on a SV20, they can just pay their amp tech $50 and it can be done pretty quickly. Marshall can (and should) do it from factory, but its dead simple for any tech to just add afterwards and isnt the sort of feature that would require a total rebuild of an amp. I'd even suggest any guitarist who is aware of PPIMV circuits, they're probably the same sort of people that have a particular preference on Plexi circuits and likely want to tweak a few values anyway. If they have power scaling, they might think "cool, extra feature for my money" and then they'll hear it and think "yeah this is barely any quieter and now the amp sounds crap" and never use it.
I think the Vintage Modern suffered somewhat by having the master volume, because people see it and naturally assume its not going to have any tonal detriment, so they set it lower. And when you do that, suddenly the tone suffers a bit, the presence doesn't work correctly and all of a sudden "its not a real Marshall any more". So in a lot of those cases, where the majority of guitarists aren't going to be technically aware of this stuff, it probably just leads to more problems than it solves.
I'm a big fan of the Vintage Modern, but I think there was a ton of mistakes with it - the name is misleading, the look is wrong (should have looked like a JTM45), HDR/LDR isn't really clear, it wasn't clear enough that its a single channel amp with 2 modes. Calling it JTM Modern or JTM Mod or something, and keeping the controls familiar would have fared a bit better (perhaps). I'd be surprised if Marshall brought it back, but maybe their Vietnamese factory gives them a bit more leeway for trying things and testing the waters.
On the whole, I think the whole mentality of "I want a nicely quiet amp, with a good master volume; I KNOW I'LL BUY A PLEXI" just makes no sense. Its a sum of the parts, its meant to be loud. If you want to trade off a bit of the pure tone of it, there's other amp circuits that do a variation better than just adding power scaling and PPIMV (which always have trade offs).