You know a large number of commercial routers run on Linux, right? The Linux kernel isn't some magic sauce that makes you immune to hacking. On the contrary, we see flaws in programs that run on Linux all the time, these being one of them. An exploit like this can work on anything, it isn't limited just to prepackaged routers.
So what you mean is get an x64 system and run a Linux distro, with some built in tools for configuring routing. Ok... So long as it doesn't have any bugs they can exploit or check for, you are fine. If it does, well then you are back to having to update... if an update is available. A lot of the router-type Linux distros aren't very well maintained. Smoothwall, the one I hear the most crowing about, had its last release in 2014.
If you were going to point to something freely available, BSD would probably be a better bet in the form of PFSense as it is actually maintained and supported pretty well. Of course the fact that it runs on BSD is incidental to its security, it is (as best we know) secure because it has competent programmers who maintain it regularly.
However the real problem is that for many people, this is just not affordable. When you try and do all your routing and filtering in software on an x64 chip, you find you need a lot of power to push traffic. The CPUs aren't designed with routing in mind so they aren't super fast at it. PFSense needs about a 2.4GHz 4 core atom to push a gigabit of traffic, and then only if the ruleset is reasonably simple. That's about $550 for an appliance from Netgate that can do that, and that is with no wireless. Well for $180 a Netgear R7000 will push a gig of traffic no issue, and comes with a 3x3 802.11ac radio that does 2.4 and 5ghz at the same time. Likewise an EdgeRouter Lite gets a gig and is wired only for $100. They pull that off by having chips with dedicated routing logic on board.
For normal users it also needs to be easy. A suggestion of "Assemble a computer from parts, load Linux, configure routing in text files and you are good," is totally unreasonable. Even something like buying an appliance and loading code on to it from a cold state is out of reach for most people. They need a ready-made solution.