I don’t follow Linux kernel developments too closely, but if complexity in the Linux kernel is such a significant factor, then what prevents anyone from relying

I don’t follow Linux kernel developments too closely, but if complexity in the Linux kernel is such a significant factor, then what prevents anyone from relying on a kernel without much of that complexity functionality built in (i.e., custom kernel)? Are the changes described just too integrated? It’s been a while since it built a custom kernel, but I seem to recall knobs for everything.