I find the lifespan issue particularly affects passively cooled consumer grade networking gear, modems, switches and routers. Modems I have the best luck with, though I usually buy Motorola/Arris which tend to be about the best consumer grade build you can get.
Routers and switches on the other hand, tend to first blow the electrolytic capacitors in the switch, then eventually the output electrolytic in the wall AC/DC adapter, or in some cases if you don't surge protect everything including all network gear and the client systems, a power surge will take out one port at a time.
These days, low port count (8 or fewer), consumer grade gigabit switches are fairly inexpensive so I just run them 24/7 as-is, but routers I also run 24/7 but do a bit more... voiding the warranty in the process.
1) Replace all electrolytic capacitors in the router with solid/polymer types. I do not wait until a failure to do this. I do it before first deployed and have never had a single solid cap fail, which it used to be fairly routine that the electrolytic caps would fail after roughly 5 years of 24/7 service.
2) Vent holes. Pop the casing off, lay down some masking tape on it, use a ruler to draw a grid where you want to put vent holes. Throw the casing on a drill press to make evenly spaced, true holes. chamfer the holes on both sides to get rid of plastic remnants.
In some cases it can make for an interesting visual effect of the front indicator LED light coming out the top.