I wish you weren't getting downvoted, but what you said is absolutely correct. If it's a module and separate from core, who cares. If it's modifying core kernel or mixed up with kernel space code, then nope.
Linux isn't a microkernel; a filesystem accepted into the kernel source tree will run as kernel space code whether or not it's compiled as a loadable module. So all 27k lines need to be audited for security purposes and to ensure they're interfacing with the rest of the kernel only in the approved ways, because there aren't a lot of technological barriers to the filesystem misbehaving.
But more important than that is the maintenance burden. NTFS will be around for a long time, but it's also a moving target because Microsoft hasn't replaced it yet. Kernel developers have to keep in mind how this code will look in a few decades, after all the original developers are retired. If it's written in a very different style from other Linux filesystems, will there be anyone left who both knows enough about the workings of the Linux IO stack 20 years hence, and understands Paragon's code conventions?
