I am afraid that sharing my personal opinion that I don't see that much use in this feature seems not to be very helpful at first glance as people have different needs and wants and views and so on. Though I'd like to say it anyway for some reasons.
One is, that I find that planing a modular system is way more complicated than planing a 19" rack due to the lack of standards for module sizes, the sometimes cumbersometopic of power consumption and the complextiy of modular systems, like how many VCA's one would need and if there are enough modulation sources than. Ups, no room for oscillators anymore...
A second thought is that I am not sure if people really plan their 19" rack in that way? At least it is easier because devices usually only differ in a minor amount of Heigth Units and mostly are designed as independent entities.
So why do I bother about something I am not interesting in? Basically because I am a bit afraid that it will consume a lot of work that would be better invested in maintaining the current state and my personal impression in other situations was, that when an idea is worded in a community and just a few people love it, things get momentum, things get more and more complicated, like troublesome and expensive to maintain and only a few people are actually using it but of course the majority of people would not participates with their views to prevent sounding negative.
As long as I open racks and suddenly modules are missing which I might find beneath other modules later, I would prefer prioritizing maintance over extension.