You might want to consider running Manjaro[0] or Haiku[1] instead (I've had great luck with both on older MacBooks [2,3]).
Nessus reports El Capitan as a "Critical" vulnerability due to lack of security updates:
According to its self-reported version number, the Unix operating system running on the remote host is no longer supported.
Lack of support implies that no new security patches for the product will be released by the vendor. As a result, it is likely to contain security vulnerabilities.
...
Mac OS X 10.11.6 (intel) support ended.
Upgrade to Mac OS X 10.14 / 10.13 / 10.12.
[2] https://tinyapps.org/blog/201811010700_linux_for_2009_macboo...
It's not super easy to replace, but if you're comfortable with a screw driver, it shouldn't pose much of a challenge, and the instruction from ifixit are great.
I'm still holding tight to my 2012 MBP, but it's got a replacement battery, and a roomier SSD to breathe some new life into it.
Say what you want about Apple and price, but I've had PCs since 1994 and the two Macs I've had have (usefully) outlasted every other machine by quite some margin - this one in particular. 6 years without formatting a Windows machine (I can't talk for now, but especially back then) would be crazy.