>including much more of the system within their scope: they are not merely kernels, but also system libraries and commands.
giving limited resources of the dev team it may lead to limited support of the system outside of the narrow set of officially supported/certified hardware with that support falling behind on modern hardware, as it happened with Sun, and vendor lock-in as a result into overpriced and low performing hardware.
There is a reason that back then at Solaris dev there was a joke about embedding Linux kernel as a universal driver for Solaris kernel in order to get reasonable support for the hardware around.