One goal of creating UBI layout was to have the router boot up in a way that does not involve any write operation on the flash. Ie. other than the stock layout which uses a block on flash to indicate if the previous boot has succeeded (and naturally this block has a limited number of writes, supposedly somewhere in the thousands, but that's statistics...), the UBI layout uses PSTORE/ramoops for it's dual-boot decision.
Hence, when using the UBI layout, a daily reboot should not reduce the lifetime of the device in any significant way.
PS: Looking at the board wearing my expected-life-time lenses, I suspect the electrolyte capacitors surrounded by the heatsink (and hence always nicely heated) would be the first components to fail. And like for every electronic device, keeping it in a dry and well-ventilated place with constant moderate temperature around 15-18 deg Celsius is ideal. To reduce heat even more at the cost of some (hardly noticeable) performance losses it's advisable to enable CPU frequency scaling which reportedly reduced the temperature by a few degrees. I have added these lines to my /etc/rc.local to setup the ondemand scaling governor by default:
echo 437500 > /sys/devices/system/cpu/cpufreq/policy0/scaling_min_freq
echo "ondemand" > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
Setting the minimum frequency is needed as when running on the lowest otherwise (wrongly) defined operating point the device fails to reboot (stuck in memory calibartion in bl2, the only remaining closed-source part...).
According to one MediaTek developer, the MT7622 is actually not specified to run at any other frequency than the maxium of 1.35GHz. However, as other MediaTek employees have added the more power-saving lower operating points and all of the apart from the (obviously wrong) lowest one work, I'm using 437.5MHz@1.0V as lowest operating point.