TBH I would expect something like this to be the next evolution of IOs. I saw mention that there was something called Global Inventions coming. I could be wrong, either on the name or on a new IO entirely, but they could well be something like what you're talking about.
I would expect them to be something account bound, that you could unslot and email to your other characters, or at least earn on one and send to another if not unslot.
I would not expect them to be commonplace, but not so rare as Purples.
Short answer is that it's the way the system was designed, as with SOs the intent is that as you level up you replace your IOs. The difference is that while with SOs it's mandatory and doesn't increase your power with IOs it does increase your power level.
The other side is that people don't always want their IOs to scale up. A lot of people equip lower level IOs so that they keep the set bonuses when exemplaring down.
Basically if they did that then IO's would be too perfect. It would seriously need some sort of disadvantage to balance that out.
Also, honestly, equipment upgrading is a staple activity not easily replaced in many games. It gives players something to shoot for, to do.
If you're going to have a loot system, the implication is that you want people to strive for better loot. Loot that autoimproves seems to defeat this design goal.
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So you think you're a hero, huh.
@Boltcutter in game.
Keep in mind that we're coming from a system in which every enhancement had to be replaced every few levels or it turned red and stopped providing any benefit at all. Having enhancements which continue to function no matter how far you outlevel them was a big change.
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I recognize that some Invention Sets end at 30 and 40, but why not just make a single IO that scales with the character?
For example, If I acquired a Karma: Def/End at level 10, why not just let it scale with me all the way to its endcap? (30 in this example). What's the intention for requiring a level 30 to replace a level 10? Just make one, and let it level up with the player.
I know this affects the economy (and the salvage reqs) needs for building, but the system seems unecessary. And on a side note, if you get a recipe that has a process (for example, Karma: Knockback Protection) it functions the same at all levels.
(I guess I was more curious than anything else, this isn't really a complaint.)