How do you know that? Why do you think that? How do you think you can do that?
The Linux kernel has over 15 million lines of code and has been created by top experts in pretty much every field and is being audited (checked for security issues) 24/7 by the top men in computer science.
Why would one rewrite 15 million lines of code instead of fixing the "issues" in the old one. Furthermore WHAT are the security problems you mean. Name examples.
No. NO! NO!!! If you'd know what you are talking about you'd know that's wrong. The biggest contributor to the kernel are companies like Intel, Red Hat, IBM who fund their employees to work and improve the kernel as well as look for security issues constantly. Those three companies contributions already make up 23% of all changes/additions. The people these companies hire are highly trained and very well educated professionals who know more about the things they do then the people which call themselfes "security experts".
See here for more: https://www.linux.com/blog/top-10-developers-and-companies-contributing-linux-kernel-2015-2016
Where do you get your "information" from? It's all wrong.
Because contributions aren't checked before entering the kernel by rigerous standards and again HIGHLY TRAINED AND PAID PROFESSIONALS WORKING FOR BIG COMPANIES WHO NEED THE KERNEL TO BE SAFE: https://www.kernel.org/doc/Documentation/SubmittingPatches
Furthermore, if "anyone" could "embed a backdoor straight into the Linux kernel" then tomorrow your Internet and bank account would stop working because those systems all need the kernel to be as safe as possible. If changes where that easy, people would've messed with it a long time ago already to make financial gain.
Have a nice day,