My question is the Kernel's complexity with over 15 million lines of code... How is it going to be able to be debugged by developers? Isn't it getting too complex? How can it sustain itself? Isn't it time for a trim down instead of just keep adding and adding?

No, not at all. The kernel you're actually *running* is nowhere near 15M lines of code. The code base seems big because it runs on so many hardware platforms and supports so many peripheral devices, file systems, etc. Most if it never gets compiled for any given installation.

Everything will be fine.

15 million lines of code - that is a burden guranteed. But then also when you look at the way the kernel is designed it is easy to debug the kernel.
Also you are forgetting the fact there are possibly thousands of developers working on the kernel. Ofcourse, there is this danger of a bug creeping in - that is the reason why stable distros (slackware, debian, redhat etc) use only the tested and known good kernels.
So unless you go out of the way and install a kernel that is a beta or a release candidate I am pretty sure you are good to go

"They're good ... damned good ..."

Ommmm.....