When people say that the Linux kernel is monolithic they really mean that it is not a microkernel architecture. In microkernels the designer says "Having a minimal microkernel with lots of separate modules is good therefore we will do everything that way". Linux simply says that modularity is good therefore we will use the appropriate integration technique at the appropriate times.
In the Linux kernel, you use printk to print messages on the console. That is modularity. There are device drivers. That is modularity. There is a range of loadable/configurable kernel modules for many things and these used to be more visible when more people would configure and build customized kernels. The Linux kernel has far more of these modules than earlier OSes that I used (TI DX10, UNIX 6th ed., Xenix, 3B2 Unix, SCO UNIX). The Linux kernel is linked into a monolithic binary that runs in kernel mode, but it is composed of many modules, some of which are integrated at link time and some of which are loaded dynamically (lsmod).
There is a good reason why the kernel is more monolithic than a business app, and that is that the kernel is doing a vastly different job at a vastly different layer of abstraction than a business app. You might also note that there are still lots of jobs for C programmers but most of them mention "embedded systems". That's what the Linux kernel is, a big featureful embedded system.
Perhaps some day someone will write a book on integration and cover all the different ways in which functionality can be integrated to produce an application. Most developers lean far too much on only one way of doing it, i.e. the link editor. For most apps, loosely coupled integration techniques are more valuable.
