Ummm, if you bothered more than a cursory glance at my thing you'd notice I AM advocating open solutions. Monowall is FreeBSD, with some mods and a nice WebUI stuck on it for configuration. EdgeOS, that runs on the ERL, is a fork of Vayetta, which is a fork/mod of Debian.
Both are open solutions but both are under active development and support by a team. Hence I'm a pretty big fan. Monowall was last updated in January, and they still support their legacy version for old hardware like WRAP systems, and their new version for more powerful systems. EdgeOS was updated in March, and they have an alpha for the next version going you can opt in to.
On the other hand the OSS firmwares are half-abandoned it seems. When I Google for Tomato I get a page that talks about it as a WRT54G firmware and looks like it hasn't seen updates in 5-8 years. Further down there's a "Tomato USB" mod on it that was updated in 2010 and still runs on 2.6.
This sort of thing does not engender trust in long term viability or freedom from bugs/exploits.
Also there's the issue that some of us have high speed needs. My Internet connection is 150/20mbps. So I need something that can support that. Triple stream N is pretty much the minimum (dual stream N maybe can in ideal cases) and AC is a better choice. Also the "router" part of the router needs to be able to keep up with that kind of speed, even when I've set up my firewall rules and such.
Finally you seem to confuse reliability with swappability. Sure, you can have a whole host of cheapass old routers and if one dies, put in a new one. However it is hard to do when you need more powerful, and thus expensive, hardware but also that isn't reliable, that is just having extras. I'd rather just have something that has less issues, that works for years on end with no problems, and not have to mess with it. That's what you get with something like a monowall box.
Also like I said, one component may need replacing before others. My Edgerouter Lite will last me a long time, unless it breaks, since it can handle around gigabit speeds with the setup I have (I've tested it). However if I get much faster Internet, I'll need a new cable modem, since mine is only 8x4 stream, and to go much above where I'm at you usually want 16 streams down. Likewise if my WAP is likely to get replaced sooner than the ERL, but probably not as soon as the cable modem.
I can have latest tech where I want it, older tech where I don't and it is all good. Also in my experience setups like that are extremely reliable.