I can’t say I’ve ever heard or read that lack of use reduces the life of a laser tube. The general take I have is that it will age, either while it sits idle or while in use. Using it at an excessive power level will eventually ruin the tube, but that’s the other end of the spectrum.
I have a 60 Watt laser and occasionally wish I had more power to make cutting cleaner, faster, easier. From my reading, for engraving, one can do well enough with up to 100 Watts, but beyond that, you lose the lower end for delicate engraving.
60 Watts is good for glass, not so much for metal. One can engrave anodized aluminum with 60 Watts and can mark metals with 60 Watts, but engraving other metals will require a fiber laser.
If you’re uncertain, aim for a true 80 Watt laser or a not-so-true 100 Watt machine. I say “not-so-true” because my alleged 60 Watt machine came in closer to the high forties until I replaced the tube with a real 60 Watt zapper.
It’s tough to determine if you are getting the real numbers. The diameter and length of the tube is information rarely provided, yet that’s the way to know.
Best advice: as much power as you can afford and fit (the higher wattage tubes are larger and usually go into larger form factor machines). With this in mind, look for a Reci W6 and that will most likely be 135 - 150 watts in a 900 x 600 machine.
I do not agree with the claims that higher wattage equals less engraving detail in any meaningful / observable result 40 watt tube vs 130 watt tube. If delicate and jaw dropping engraving is what you’re after, you’d buy a diode or fiber laser.