Every invest-vs-reward is an "economy". How much is a repair kit worth? Basically nothing, because you find tons of them and also can craft them easily, same applies to executing a repair process (because the doing itself is quite pointless). That's economy.
If anything is massively under- or overrated, it's a bad economy.
And there are dependencies of that. If repairing wouldn't be that "cheap", it would increase the value of found items. So once you have a found a t3 ak-47 and find another one, there is only few you can do with it. Selling it to the trader probably is done in the most cases. Assuming you are able to craft a higher tier AK, you also might scrap it for parts.
But if repairing wouldn't be that cheap or had other backdraws, there is another valuable option: Keep it as a backup.
Sure, you could call that an economy in the broadest sense of the word, but I'm not sure that nomenclature really helps anyone as most people understand economy as having to do with the exchange of goods and/or services between individuals - in this case game entities whether PC or NPC.
I assert that this game does not have an economy as such, because by default settings the supply of every resource is theoretically endless, capped only by time and RNG. This means that on a long enough timeline the demand, and hence the value, for everything drops to zero. You cannot have any kind of meaningful economy in this situation.
The solution to that involves item sinks, i.e. consistent, recurring ways to remove items from circulation. The OP pinpoints repair kits as being responsible for keeping items in circulation and suggests their removal or nerfing as a direction for creating an item sink through degradation. What the OP completely ignores is the question of whether this game actually benefits from this kind of "economy".
You put forward keeping loot as a backup as a valuable option, but I question the assumption of your premise. As it is now, you keep the best version of a particular item and use it, scrapping or selling all inferior versions. Perhaps in the rare, odd case you would find two versions with stats divergent enough from each other that you would keep both and use them for different purposes. So let's now add this concept of degradation and keeping backups...how does that actually feel to the player when they are forced to go from their high stat item to an inferior version? What is the counterplay to these downgrades over time?
In my experience, finding upgrades in the mid-late game is rare. Once you have a blue or purple version of an item it can take weeks to upgrade over it, if ever. If I were not able to keep my best versions of items going indefinitely, this would mean fairly frequent statistical downgrades and less frequent upgrades. That sounds like a bad feeling game, IMO, where you slowly get weaker over time as the game actually gets harder.
I think the repair mechanic as it exists is perfectly adequate. It's not super engaging, but it does feed a gameplay loop that feels good. You hunt new items to improve on the ones you have, rarely finding them, but when you do it feels nice. The rest of the time you are collecting vendor fodder which increase your liquid wealth, also lending a feeling of progression. If you find enough of these redundant items, you build up a nice cash reserve which can be used to fill in any supply gaps you may have. As your reserve grows, you are motivated to explore farther to find more traders and add them to your rotation, checking them for useful items every time they refresh their stock.
In absence of a true endgame, this works pretty well for now, IMO. I don't see how adding "feels bad" mechanics for the sake of an imaginary "economy" improves the game.
Sure, a glass of water in the desert is priceless. But a glass of water in the desert is still priceless when you went past an oasis 10 hours earlier and forgot to fill up.
Repair kits are so easy to get that the blame is on you for making them artificially valuable
I honestly don't understand what you are trying to say here.