I think one big advantage could be seemless integration of all applications. Genera on Lisp Machines [1] and McClim [2] illustrate how it works.

[1] https://news.ycombinator.com/item?id=1878220

[2] https://common-lisp.net/project/mcclim/excite.html

In classical OS (Linux, Windows, Mac) the developer is responsible for seemless integration (in KDE or MS Office for instance). It is easy to break design guidelines here which causes ugly interoperation, or even lack of interoperation.

I don't know. Why do people still write from-scratch OS's in C when there's a perfectly good Linux? Why does musl exist?

> Why does musl exist

At least in part, because of storage and memory constrained things like home routers.