If I remember correctly, Modulex was created in the 60’s as a tool for architects to visualize building designs. I don’t know how successful it was in that regard.
There’s a couple people in the AFOL world who are super into Modulex, and one year at BrickFair our goodie bag contained a sample of Modulex bricks, I believe supplied by one of the people who’s really into them and had some Modulex stuff on display that year. I’m sure I still have mine somewhere.
I have zero interest in Modulex. They’re a different scale than regular Lego pieces and so they don’t attach at all. They’re old and uncommon so they’re expensive and hard to find used, and I’m not sure you can find them new at all. They do come in a neat variety of pastel colors, but the parts selection is extremely limited, and leans heavily towards 1x bricks.
I’m sure people could come up with clever ways to incorporate a modulex build into a Lego construction, and no doubt if you had a pile of them, you could build something neat. But I can’t see how anybody could sustain it as a full-time hobby or even have more than a passing interest in it when the world of Lego already exists.