Just beacuse you paid a bunch of money for it and flashed it with a better firmware doesn't make it good hardware, and custom firmwares often have poor support (and, on more than a few occasions, worse performance) than the default firmwares optimized by the manufacturer for the specific hardware they have.
The fact that you have to keep buying new hardware tells me that you have lousy hardware and should step up to proper hardware rather than expensive consumer-grade stuff.
My work expensive Cisco access points are so reliable I've never seen some of them in 10 years (they're above the dropped ceiling)
What the heck are you doing with your WiFi gear to have these problems? I've used all kinds of cheap-o routers over the years, and never had problems with them overheating. Are you living in the desert without A/C or something? My current router is just a cheap TP-Link dual-band I got used on Ebay; it runs DD-WRT and has been working fine for 1.5 years hiding behind my sofa. Before that, I had a cheap Cisco/Linksys e1000 I got used and ran DD-WRT on for probably 3 years. I gave it away to someone, and last I heard it still works just fine. Before that I had some D-Link I think, which I replaced because a firmware update prevented my network print jobs from going through (obviously not a hardware problem). I've also had (about 4 years ago, for work) a Cisco/Linksys dual-band which worked great, and a Cisco enterprise AP which was very reliable though the IOS UI was the worst thing I've ever seen in my life. Honestly, your post is the first time I've heard anyone complain about WiFi routers having overheating problems.
Maybe I'm luckier because I usually run DD-WRT and set the transmit strength a bit lower, but I've only done that with the last couple of routers.
I have another Linksys "premium" home router ($200-300) but the WiFi died on that too, but it lives on as an OpenWRT home router but with WiFi disabled. The Apple WiFi routers also lived on for a while with WiFi disabled but acting as TimeMachine backups. Then the disks died and I couldn't be arsed to pry open the case with a hair drier to get at the disk inside and replace it. So for my home, the setup went from Apple all-in-one combo gear with routing, WiFi and backup disk, to specialized functions for WiFi (cheapest off the shelf), routing (openwrt), and backup disks (standard NAS hardware). (For TimeMachine.)
I hope my 15" Macbook lasts for a really long time, because when it's dead I don't know if Apple has any Macbook I want to have. But that's tomorrows sorrows..., no use worrying about that until the time comes. (I guess I can always fall back to Linux and some PC laptop.)