Simply by going into Gameplay->Settings and changing both fDamageToArmorPercentage and fDamageToWeaponValue to zero you can completely eliminate both weapon & armor decay in the game. I mention this to show how numbingly easy this was to do in case you wish to add this effect to your own mods without even having to use this ESP at all.

There are two versions: The first one only eliminates weapon decay but leaves armor still destroyable. This is better for stealth characters who don't get hit much (if at all) but don't want to have to max their repair skill and blow all their caps buying new weapons just for spare parts all the time simply because Bethesda has apparently never heard of a gun cleaning kit.

The second one eliminates armor decay as well. This can be a bit overpowering, but hey, you try finding spare parts for a stealth suit.

The catch with both of these is, as you expect, that this also applies to all equipment equally - including your enemies. So if you load the no armor decay version and blast some guy in power armor a thousand times it's just not going to break down on him. Thus this can theoretically make certain "boss" encounters actually more difficult.

Ideally I would have rather made an actual gun cleaning kit (see above); a reusable, non-consumable item that could only take weapons up to 100% provided that they were already over 75% functionality (being that over this 75%-100% range really would just be dirt, grime & powder residue, after all). The catch is that I'm obviously not any sort of real modder and such a project is way beyond my feeble abilities.

Feel free to do whatever you want with this. Tweak it, change it, paste it into your own thing, make it sing opera - go for it.