Yeah - the DSL is also a more complex amp than a JCM800 so should be more tricky to build and maintain. However, its made in a more modern fashion in Vietnam. There are design choices throughout the amp aimed at getting the cost down.
They could absolutely make a Vietnamese build JCM800, but would then people would say "oh it should be made in England with the original spec transformers". Marshall themselves constantly adjusted how they built amps to get costs down and numbers up, and its why certain amps from a particular series are more desirable than others. (PCM vs turretboard Superleads, horizontal input vs vertical input 800's).
I'm not sure that the assumption about profit margins would be any different between the amps - I think building amps in England with the parts they use just inflates the price massively. I'm sure its as cheap as they can possibly do it while making it worthwhile in the first place. I wouldnt be surprised if the cheaper amps even had a bigger profit margin to them, and maybe it even helps them keep the UK factory open. Why would you assume their JCM800's or Plexi's have bigger profit margins? Only because they cost significantly more?
I think you basically nailed it with existing amps not replacing peoples desire for a real 800 or Plexi, and thats basically what they're competing with themselves against. Any new amp would have this same situation too. I think thats why it was the DSL range that got moved to Asia - it you want the tone of classic Marshall amps, but dont want to pay, thats your option. If you want the classic stuff, built in the UK and with the look of the original, you buy the reissue. The JVM is their compromise of having a modern amp built in the UK with all the bells and whistles. I think its more of an outlier in their model rather than the product line that pays the staff and the bills.
I legit think this is what Marshall have tried to do with almost every multichannel amp they've released since the 90's. Read the blurbs for 6100, DSL, TSL, JVM, Vintage Modern etc. Whether or not they achieve that, or package it in a way that is a total success is another matter.
I'd love Marshall to build amps the way Bogner or Friedman or Wizard or Headfirst or MGL or Metropolous etc do, but those companies are operating under entirely different parameters. I think bridging that gap seems easy to us as consumers but