I live in a quite a good area with a low crime rate and so I am not normally too concerned if either my house or my car is unlocked for a while. OTOH, if I leave this area and go elsewhere then I always ensure that I lock my car and keep my valuables in a hotel safe.
I regard the internet in a similar way. As long as I am using a Linux OS via my home NAT router then I feel secure as long as I don't do anything stupid, such as visit dodgy websites or install software from anywhere other than the official repos for my distro.
So far, after using Linux systems in this way for around fifteen years, this philosophy has served me well as I have never
experienced any malware of any kind, either viruses or rootkits.
I would add that, while I consider a software firewall unnecessary for a home system behind a NAT router, I do
use UFW when I am away from home as you just don't know who else may be on the line.
I do run a couple of servers, NFS and SSH, but again, as they are behind my router, I really don't worry about them.
So, for normal home/SOHO users, I do not really consider rootkits a problem unless
the user is particularly reckless!
To quote from the Wikipedia Rootkit
System hardening represents one of the first layers of defence against a rootkit, to prevent it from being able to install. Applying security patches, implementing the principle of least privilege, reducing the attack surface and installing antivirus software are some standard security best practices that are effective against all classes of malware.
As most of these factors are actually inherent to Linux design, (antivirus is not
necessary!), then all that remains is to guard against an "Evil Maid Attack
" as, if someone else has physical access to the machine, then all software defences are irrelevant!