Updating the various settings of old would be positive thing for the game, Sure, some people might not value certain campaign settings, but they all have their fan base. I really don't see why a particular setting has to be justified by popularity or potential dollar value before it can be supported. Even if support is a a single page of conversion notes, it would make fans happy. Like you said, many settings don't need a lot of mechanical work, which is why it wouldn't take much for them to be updated. It would also help to peek the interest of newer generations of gamers. It's not like releasing a sub classes and races for Taladas would require WotC to make any further commitments. The justified dollar value in this case would be increased sales of older campaign settings. Sure, there are some folks who hate certain campaign settings, but the game doesn't grow by catering to the whims of haters.
I don't think it's settings that need to be justified from a potential dollar value before it can be supported...I think that's true of anything that they do. Any product they are going to produce needs to be looked at as an investment of resources with some expectation of return.
Let's say they produce a PDF only version of "The Worlds of D&D", a short guide to many of their past campaign worlds. It could have the required mechanical aspects of each setting, such as races and subclasses, and perhaps a bare minimum of fluff info to get them started, while also siting previous works that can be found to enhance that experience.
I don't think that it would be incredibly difficult to do this. I think it would be a bit more of an investment in time than maybe you are imagining, but overall, I think it could be done. The play testing would most likely be the biggest time investment.
The question really comes down to what isn't being done while that is? Let's say that Chris Perkins does the majority of the work and then Jeremy Crawford has to proof read and vet it and then finally Mike Mearls has to review and approve it (this is probably in no way accurate to how things happen over there, but is just an example). That is all time on their part. Time that would otherwise be spent on another product. Do they scratch another product to focus on this? DO they delay the release of another product to work on this?
Those are the questions that come into play, I think.
I am pretty sure they are looking at it as "Lankhmar isn't going to hook new players, it is something that appeals to existing players" and as such, material for that is best left to the community for now.
They have limited resources, so they have to decide what would be best. My examples are pretty basic and crude, but I think that's what it boils down to...they have limited resources and have to decide how to spend those resources and maintain their production schedule and sales expectations.
I say this knowing that if they did put out a Worlds of D&D, I'd likely snatch a copy up immediately.